Recent geopolitical events have intensified debates on the norms governing territorial integrity under international law. The full-scale Russian invasion of Ukraine in 2022, Israel’s continued military activities in the occupied Palestinian territories, and Morocco’s assertive claims over Western Sahara highlight growing concerns about the erosion of the prohibition against forcible annexation. A parallel issue is emerging in the eastern Democratic Republic of the Congo (DRC), where Rwanda’s alleged involvement—through a proxy, the March 23 Movement (M23) armed non-state actor (ANSA)—raises similar alarms. The muted or inconsistent reactions of the international community, particularly from influential states and institutions, raise the critical question of whether we are witnessing an inflection point in the customary norm prohibiting forcible annexation.
The Customary Prohibition of Forcible Annexation
Customary international law is shaped by consistent state practice and the belief that such practice carries legal obligations (opinio juris). The prohibition of forcible annexation is well-established as a customary norm, yet its extent and application have been subject to increasing debate. The Declaration on Principles of International Law Concerning Friendly Relations indicates:
The territory of a State shall not be the object of military occupation resulting from the use of force in contravention of the provisions of the Charter. The territory of a State shall not be the object of acquisition by another State resulting from the threat or use of force. No territorial acquisition resulting from the threat or use of force shall be recognized as legal.
However, traditionally, it has been viewed as an extension of the broader prohibition against the use of force, enshrined in Article 2(4) of the United Nations Charter. But, scholars Brunk and Hakimi contend that conflating these norms undermines the unique protective function of the annexation prohibition: they argue that this prohibition is not merely a derivative of broader principles governing the prohibition or threat of use of force, but rather a distinct and foundational principle of international law. They emphasize—rightly in my opinion—its role in three key areas: establishing territorial titles, regulating the use of force across borders, and safeguarding self-determination. This understanding positions the prohibition as an independent norm crucial to maintaining international stability.
Most recently, the International Court of Justice (ICJ) in a rather seminal pronouncement in its 2024 Advisory Opinion on the Legal Consequences Arising from the Policies and Practices of Israel in the Occupied Palestinian Territory, including East Jerusalem, reaffirmed that the acquisition of territory by force is unequivocally prohibited under international law. The Court held that Israel’s policies—particularly the expansion of settlements, construction of infrastructure including the wall, exploitation of natural resources, and application of Israeli law in East Jerusalem and parts of the West Bank—amount to de facto annexation. It emphasized that belligerent occupation cannot confer lawful title, nor can it extinguish the rightful sovereign claims of the occupied.
Emerging Challenges to the Norm
Here is the puzzle: that the above notwithstanding, recent international responses—or lack thereof—seem to contradict the norm, raising questions about whether we are witnessing a gradual shift in customary practice. In Ukraine, for instance, ongoing discussions suggesting that Kyiv might have to concede territorial losses to Russia, which controls the Ukrainian oblasts of Luhansk, Donetsk, Zaporizhzhia and Kherson, prompt concerns about the potential weakening of the annexation prohibition. The expansion of Israeli settlements in the West Bank—despite the recent 2024 Advisory Opinion—and proposals for exclusive Israeli and American (United States) control over, development and reconstruction of Gaza, further illustrate a pattern of expansive territorial ambitions met with insufficient international pushback, with most European countries pivoting towards political narratives on the protection of a two state solution, rather than a clear outright rejection of the forcible acquisition of territory (only mentioned by Saudi Arabia, and Jordan). It deserves being mentioned that, following the ICJ decision in May 2024 calling on Israel to halt its incursion into Rafah, in southern Gaza, European foreign ministers gathered to deliberate on sanctions against Israel in the event of non-compliance. But the European Union ultimately did not move forward with sanctions. Admittedly, though this was about obligations related to the Convention against Genocide, there has similarly been no observable change in behaviour towards Israel since the 2024 Advisory Opinion, which calls on States not to not to recognize, and not to render aid or assistance in maintaining the situation created by measures that are illegal under international law, including those aimed at advancing annexation in the Occupied Palestinian Territory, including East Jerusalem, and other Arab territories occupied by Israel since 1967.
Similarly, Morocco’s claim over Western Sahara has gained increasing recognition, despite the ICJ’s 1975 Advisory Opinion dismissing its legal basis for sovereignty over the territory. In December 2020, the United States recognized Morocco’s sovereignty over Western Sahara, with Germany, Spain, and several African, Caribbean and Gulf states endorsing its autonomy plan. The gradual normalization of Morocco’s claims, through diplomatic measures such as the opening of consulates in Western Sahara, signals a trend that could influence how territorial claims are legitimized in international law.
In the context of eastern DRC, credible reports suggest that Rwanda has directly supported M23 in expanding territorial control in North and South Kivu, including in Goma and Bukavu. The presence of an estimated 6,000 Rwandan Defense Forces (RDF) troops has prompted questions about Rwanda’s long-term strategic intentions. Some analysts argue that this military presence may indicate efforts to establish a buffer zone aligned with Rwandan interests, potentially laying the groundwork for future territorial claims; a progression of events already witnessed in Ukraine, with the Donbas oblasts of Donetsk and Luhansk now being claimed by Russia, through proxy ANSAs. Given the broader context of territorial conflicts elsewhere and the existence of regressive precedent, the situation in the DRC underscores the risk that continued tolerance of such actions may erode the norm against forcible annexation.
The Process of Customary Norm Evolution
Customary law is not static; it evolves through state practice and shifting perceptions of opinio juris. The ICJ in Military and Paramilitary Activities in and against Nicaragua, has emphasized that the establishment of a customary rule does not require absolute conformity but rather general consistency in state behavior. Any deviation must be viewed as a violation of the norm rather than as an indication of normative transformation. Therefore, if repeated “violations” or conduct contrary to the existing rule are met with acquiescence or recognition by other states, the process of normative change may begin.
The Court does not consider that, for a rule to be established as customary, the corresponding practice must be in absolutely rigorous conformity with the rule. In order to deduce the existence of customary rules, the Court deems it sufficient that the conduct of States should, in general, be consistent with such rules, and that instances of State conduct inconsistent with a given rule should generally have been treated as breaches of that rule, not as indications of the recognition of a new rule. (para. 186—Emphasis added)
Essentially, the transformation of a customary norm would involve two phases: the decline of the old norm and the consolidation of a new rule. As observed in past and ongoing transitions in international law—such as the evolving stance on the legality of nuclear weapons—new norms can emerge through a dialectic process of state action, legal interpretation, and international reaction. Accordingly, in the case of territorial annexation, the lack of a strong response to certain cases of territorial acquisition could eventually be interpreted as a shift towards permissibility under specific circumstances.
The ICJ’s position on norm evolution further complicates the current trajectory. The Court has acknowledged that while widespread state practice consistent with an emerging norm could indicate the advent of a new rule, this must be accompanied by an explicit shift in opinio juris. In the Nuclear Weapons Advisory Opinion, for instance, the ICJ noted that a clear divergence between emerging state practice and established norms can result in a transitional phase where competing norms coexist until one is decisively recognized.
Having said this, the Court points out that the adoption each year by the General Assembly, by a large majority, of resolutions recalling the content of resolution 1653 (XVI), and requesting the member States to conclude a convention prohibiting the use of nuclear weapons in any circumstance, reveals the desire of a very large section of the international community to take, by a specific and express prohibition of the use of nuclear weapons, a significant step forward along the road to complete nuclear disarmament. The emergence, as lex lata, of a customary rule specifically prohibiting the use of nuclear weapons as such is hampered by the continuing tensions between the nascent opinio juris on the one hand, and the still strong adherence to the practice of deterrence on the other. (para. 73—Emphasis added)
The current geopolitical landscape suggests that the prohibition of annexation may be entering such a phase, with competing state conduct challenging its absolute character. The situation described by the ICJ regarding nuclear weapons provides a useful analogy for assessing the current state of the prohibition against forcible annexation in the face of emerging gradual annexation practices. Just as the persistence of nuclear deterrence practices has hindered the full emergence of a customary prohibition against nuclear weapons, the ongoing and largely unchallenged incremental territorial acquisitions—such as those seen in the occupied Palestinian territories, parts of Ukraine, and potentially eastern DRC—create a tension between the existing prohibition on forcible annexation and the evolving reality of state practice. While there is no explicit shift in opinio juris affirming the legality of annexation, the international community’s inconsistent responses, muted objections, and selective recognition of territorial acquisitions risk undermining the norm’s absolute character. This could lead to a transitional phase where both the prohibition and contrary state practices coexist, thereby diluting the strength of the existing customary norm without yet fully consolidating a new one. Consequently, if these practices continue without decisive rejection, they may, over time, erode the perception of annexation as an absolute violation of international law, just as the deterrence-based approach to nuclear weapons has prevented a clear legal prohibition from fully crystallizing.
Implications and the Need for Vigilance
The potential erosion of the prohibition against forcible annexation carries profound implications for international stability. If the norm weakens, states with expansionist ambitions may feel emboldened to pursue territorial gains through force, leading to increased conflicts and destabilization. Moreover, weakening this prohibition risks undermining self-determination, particularly in contested regions where communities seek to establish or maintain political autonomy.
Maintaining the distinct identity of the prohibition against forcible annexation is crucial. As Brunk and Hakimi argue, it must not be conflated with the general prohibition on the use of force, and should, conceivably, be considered as attaining the peremptory status of jus cogens. While Article 2(4) of the UN Charter addresses broader issues of military aggression, the annexation prohibition plays a specific role in protecting territorial integrity and preventing de facto sovereignty changes through coercion. A clear legal distinction—including of peremptory nature—ensures that states cannot justify annexation by referencing self-defense or other lawful uses of force.
Given the stress placed on this norm in contemporary geopolitics, bolstering its absolute nature is imperative. This requires sustained diplomatic pressure, legal reaffirmation in international forums, and consistent state practice opposing territorial annexations. The role of international courts, treaty bodies, and multilateral organizations will be critical in resisting attempts to redefine, reverse, or dilute the prohibition.
Conclusion
Recent events suggest that the prohibition against forcible annexation is under significant pressure. While it remains a customary norm, emerging state practices and inconsistent international responses indicate a potential shift in its application. The situation in Ukraine, Palestine, Western Sahara, and the eastern DRC exemplifies the risks of a normative transition towards a more permissive stance on territorial acquisition. If left unchallenged, such developments could fundamentally alter the principles governing state sovereignty and territorial integrity.
To prevent this erosion, the international community must reinforce the prohibition as a distinct and inviolable norm. Legal scholars, policymakers, and international institutions must remain vigilant in upholding territorial integrity and resisting attempts to normalize annexation through force. Ensuring that the prohibition remains robust and widely respected is essential for preserving global stability and maintaining the foundational principles of modern international law.
Territorial Annexation and Custom: Are we at an Inflection Point in the System of International Law and Relations?
Written by Joshua Joseph NiyoRecent geopolitical events have intensified debates on the norms governing territorial integrity under international law. The full-scale Russian invasion of Ukraine in 2022, Israel’s continued military activities in the occupied Palestinian territories, and Morocco’s assertive claims over Western Sahara highlight growing concerns about the erosion of the prohibition against forcible annexation. A parallel issue is emerging in the eastern Democratic Republic of the Congo (DRC), where Rwanda’s alleged involvement—through a proxy, the March 23 Movement (M23) armed non-state actor (ANSA)—raises similar alarms. The muted or inconsistent reactions of the international community, particularly from influential states and institutions, raise the critical question of whether we are witnessing an inflection point in the customary norm prohibiting forcible annexation.
The Customary Prohibition of Forcible Annexation
Customary international law is shaped by consistent state practice and the belief that such practice carries legal obligations (opinio juris). The prohibition of forcible annexation is well-established as a customary norm, yet its extent and application have been subject to increasing debate. The Declaration on Principles of International Law Concerning Friendly Relations indicates:
However, traditionally, it has been viewed as an extension of the broader prohibition against the use of force, enshrined in Article 2(4) of the United Nations Charter. But, scholars Brunk and Hakimi contend that conflating these norms undermines the unique protective function of the annexation prohibition: they argue that this prohibition is not merely a derivative of broader principles governing the prohibition or threat of use of force, but rather a distinct and foundational principle of international law. They emphasize—rightly in my opinion—its role in three key areas: establishing territorial titles, regulating the use of force across borders, and safeguarding self-determination. This understanding positions the prohibition as an independent norm crucial to maintaining international stability.
Most recently, the International Court of Justice (ICJ) in a rather seminal pronouncement in its 2024 Advisory Opinion on the Legal Consequences Arising from the Policies and Practices of Israel in the Occupied Palestinian Territory, including East Jerusalem, reaffirmed that the acquisition of territory by force is unequivocally prohibited under international law. The Court held that Israel’s policies—particularly the expansion of settlements, construction of infrastructure including the wall, exploitation of natural resources, and application of Israeli law in East Jerusalem and parts of the West Bank—amount to de facto annexation. It emphasized that belligerent occupation cannot confer lawful title, nor can it extinguish the rightful sovereign claims of the occupied.
Emerging Challenges to the Norm
Here is the puzzle: that the above notwithstanding, recent international responses—or lack thereof—seem to contradict the norm, raising questions about whether we are witnessing a gradual shift in customary practice. In Ukraine, for instance, ongoing discussions suggesting that Kyiv might have to concede territorial losses to Russia, which controls the Ukrainian oblasts of Luhansk, Donetsk, Zaporizhzhia and Kherson, prompt concerns about the potential weakening of the annexation prohibition. The expansion of Israeli settlements in the West Bank—despite the recent 2024 Advisory Opinion—and proposals for exclusive Israeli and American (United States) control over, development and reconstruction of Gaza, further illustrate a pattern of expansive territorial ambitions met with insufficient international pushback, with most European countries pivoting towards political narratives on the protection of a two state solution, rather than a clear outright rejection of the forcible acquisition of territory (only mentioned by Saudi Arabia, and Jordan). It deserves being mentioned that, following the ICJ decision in May 2024 calling on Israel to halt its incursion into Rafah, in southern Gaza, European foreign ministers gathered to deliberate on sanctions against Israel in the event of non-compliance. But the European Union ultimately did not move forward with sanctions. Admittedly, though this was about obligations related to the Convention against Genocide, there has similarly been no observable change in behaviour towards Israel since the 2024 Advisory Opinion, which calls on States not to not to recognize, and not to render aid or assistance in maintaining the situation created by measures that are illegal under international law, including those aimed at advancing annexation in the Occupied Palestinian Territory, including East Jerusalem, and other Arab territories occupied by Israel since 1967.
Similarly, Morocco’s claim over Western Sahara has gained increasing recognition, despite the ICJ’s 1975 Advisory Opinion dismissing its legal basis for sovereignty over the territory. In December 2020, the United States recognized Morocco’s sovereignty over Western Sahara, with Germany, Spain, and several African, Caribbean and Gulf states endorsing its autonomy plan. The gradual normalization of Morocco’s claims, through diplomatic measures such as the opening of consulates in Western Sahara, signals a trend that could influence how territorial claims are legitimized in international law.
In the context of eastern DRC, credible reports suggest that Rwanda has directly supported M23 in expanding territorial control in North and South Kivu, including in Goma and Bukavu. The presence of an estimated 6,000 Rwandan Defense Forces (RDF) troops has prompted questions about Rwanda’s long-term strategic intentions. Some analysts argue that this military presence may indicate efforts to establish a buffer zone aligned with Rwandan interests, potentially laying the groundwork for future territorial claims; a progression of events already witnessed in Ukraine, with the Donbas oblasts of Donetsk and Luhansk now being claimed by Russia, through proxy ANSAs. Given the broader context of territorial conflicts elsewhere and the existence of regressive precedent, the situation in the DRC underscores the risk that continued tolerance of such actions may erode the norm against forcible annexation.
The Process of Customary Norm Evolution
Customary law is not static; it evolves through state practice and shifting perceptions of opinio juris. The ICJ in Military and Paramilitary Activities in and against Nicaragua, has emphasized that the establishment of a customary rule does not require absolute conformity but rather general consistency in state behavior. Any deviation must be viewed as a violation of the norm rather than as an indication of normative transformation. Therefore, if repeated “violations” or conduct contrary to the existing rule are met with acquiescence or recognition by other states, the process of normative change may begin.
Essentially, the transformation of a customary norm would involve two phases: the decline of the old norm and the consolidation of a new rule. As observed in past and ongoing transitions in international law—such as the evolving stance on the legality of nuclear weapons—new norms can emerge through a dialectic process of state action, legal interpretation, and international reaction. Accordingly, in the case of territorial annexation, the lack of a strong response to certain cases of territorial acquisition could eventually be interpreted as a shift towards permissibility under specific circumstances.
The ICJ’s position on norm evolution further complicates the current trajectory. The Court has acknowledged that while widespread state practice consistent with an emerging norm could indicate the advent of a new rule, this must be accompanied by an explicit shift in opinio juris. In the Nuclear Weapons Advisory Opinion, for instance, the ICJ noted that a clear divergence between emerging state practice and established norms can result in a transitional phase where competing norms coexist until one is decisively recognized.
The current geopolitical landscape suggests that the prohibition of annexation may be entering such a phase, with competing state conduct challenging its absolute character. The situation described by the ICJ regarding nuclear weapons provides a useful analogy for assessing the current state of the prohibition against forcible annexation in the face of emerging gradual annexation practices. Just as the persistence of nuclear deterrence practices has hindered the full emergence of a customary prohibition against nuclear weapons, the ongoing and largely unchallenged incremental territorial acquisitions—such as those seen in the occupied Palestinian territories, parts of Ukraine, and potentially eastern DRC—create a tension between the existing prohibition on forcible annexation and the evolving reality of state practice. While there is no explicit shift in opinio juris affirming the legality of annexation, the international community’s inconsistent responses, muted objections, and selective recognition of territorial acquisitions risk undermining the norm’s absolute character. This could lead to a transitional phase where both the prohibition and contrary state practices coexist, thereby diluting the strength of the existing customary norm without yet fully consolidating a new one. Consequently, if these practices continue without decisive rejection, they may, over time, erode the perception of annexation as an absolute violation of international law, just as the deterrence-based approach to nuclear weapons has prevented a clear legal prohibition from fully crystallizing.
Implications and the Need for Vigilance
The potential erosion of the prohibition against forcible annexation carries profound implications for international stability. If the norm weakens, states with expansionist ambitions may feel emboldened to pursue territorial gains through force, leading to increased conflicts and destabilization. Moreover, weakening this prohibition risks undermining self-determination, particularly in contested regions where communities seek to establish or maintain political autonomy.
Maintaining the distinct identity of the prohibition against forcible annexation is crucial. As Brunk and Hakimi argue, it must not be conflated with the general prohibition on the use of force, and should, conceivably, be considered as attaining the peremptory status of jus cogens. While Article 2(4) of the UN Charter addresses broader issues of military aggression, the annexation prohibition plays a specific role in protecting territorial integrity and preventing de facto sovereignty changes through coercion. A clear legal distinction—including of peremptory nature—ensures that states cannot justify annexation by referencing self-defense or other lawful uses of force.
Given the stress placed on this norm in contemporary geopolitics, bolstering its absolute nature is imperative. This requires sustained diplomatic pressure, legal reaffirmation in international forums, and consistent state practice opposing territorial annexations. The role of international courts, treaty bodies, and multilateral organizations will be critical in resisting attempts to redefine, reverse, or dilute the prohibition.
Conclusion
Recent events suggest that the prohibition against forcible annexation is under significant pressure. While it remains a customary norm, emerging state practices and inconsistent international responses indicate a potential shift in its application. The situation in Ukraine, Palestine, Western Sahara, and the eastern DRC exemplifies the risks of a normative transition towards a more permissive stance on territorial acquisition. If left unchallenged, such developments could fundamentally alter the principles governing state sovereignty and territorial integrity.
To prevent this erosion, the international community must reinforce the prohibition as a distinct and inviolable norm. Legal scholars, policymakers, and international institutions must remain vigilant in upholding territorial integrity and resisting attempts to normalize annexation through force. Ensuring that the prohibition remains robust and widely respected is essential for preserving global stability and maintaining the foundational principles of modern international law.
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Andrea Pelliconi says
May 3, 2025
Thanks for the very excellent analysis. I agree with the sentiment of the conclusion and endorse the call for strong stances against territorial annexation. In my opinion, the ICJ's position (including the most recent advisory opinion on Israel and Palestine) alongside the state submissions as indication that opinio juris on the topic hasn't changed; the EU and individual states have continued to issue statements where, at least in words, reiterate their confrontation for unilateral annexation. The situation is thus tricky but I feel that we can still say that states don't hold the opinion that it is lawful to unilaterally annex a territory of another state - at the end of the day, they wouldn't want this for themselves. In addition, both Western Sahara and the Ukrainian territories are not *portrayed* by Morocco and Russia as annexation. Morocco has consistently held that Western Sahara has always been part of Morocco's sovereign territory, while Russia has - on the face of it - not annexed the territory but integrated what it framed as independent states (the neo-declared Republics that it recognized three days before invading Ukraine). These gymnastics are, in my view, exactly aimed at avoiding the claim that unilateral annexation is acceptable. As a last point, I would tend to make the argument that the opinio juris standard for the purpose of creating a new customary norm (e.g. prohibition of nuclear weapons) is not the same as that for a change in an existing customary rule. In other words, proving that something isn't customary anymore should be subject to a higher threshold and can certainly not be inferred by somewhat widespread practice against the rule (especially if not coupled with explicit endorsements of a new rule by the vast majority of states). These are just some reflections stimulated by your excellent post, which has very similar conclusions to mine, so I look forward to further discussions and inputs on such a fundamental issue
Joshua says
June 9, 2025
Many thanks for your thoughtful engagement, Andrea, and sincere apologies for the late response — I only just came across your comment and have now had a moment to respond.
I truly appreciate your kind words about the piece. I’m especially glad that the general sentiment resonated with you, as the principal aim of the commentary was precisely to underline the need for a stronger, more principled, and more consistent stance by the international community against the forcible annexation of territory.
To your thoughtful reflections:
On the consistency of opinio juris as affirmed by the ICJ — I agree entirely that the ICJ’s recent advisory opinions, particularly the one concerning Israel and the Occupied Palestinian Territory, reflect a continuity in the legal position that unilateral annexation remains impermissible. That is why I made a point of referencing them. My concern, however, is not that opinio juris has definitively changed, but rather that the absence of near-unanimous affirmation—especially on issues so fundamental to the international legal order—risks undermining the clarity and normative force of the rule. As you rightly note, self-interest generally supports adherence to the principle of territorial integrity. But for the more powerful states—those unlikely to be on the receiving end of annexation—the calculus may be different. For them, legal ambiguity (or even erosion) may serve their geopolitical interests.
On declaratory statements versus practice — This is exactly where the dissonance becomes most concerning. While many states, including the EU and several Western governments, have issued statements opposing annexation, their actual practice—particularly the robustness (or lack thereof) of their responses to violations—often fails to match the rhetoric. It is this gap between word and deed that creates the conditions for normative decline, even if the formal elements of custom (state practice and opinio juris) are not yet fully displaced. In this sense, I am not arguing that a new norm is emerging, but that we may be entering a period of inflection, where the stabilizing force of the existing norm is weakening under the weight of inconsistent responses.
On the threshold for change in customary international law — I fully agree that changing or displacing an existing customary norm, especially one so central to the international legal order, demands a higher threshold than the creation of a new norm. And rightly so. The prohibition on annexation is closely bound up with the principle of territorial integrity and arguably approaching the realm of jus cogens. But therein lies a danger: over-reliance on the idea that such norms are immutable can breed complacency. In practice, this can allow for gradual erosion through inconsistent enforcement and rhetorical hedging, even if the formal legal standard remains unchanged. During the intervening period of inconsistency, the protective function of the norm may become significantly diminished.
On the “legal gymnastics” employed by annexing powers — You’ve captured this perfectly. The rhetorical and legal contortions—whether in Russia’s invocation of newly recognized republics or Morocco’s historical claim over Western Sahara—seem calculated to obscure what are, in substance, acts of annexation. These manoeuvres are not persuasive, and I struggle to treat them as legitimate legal arguments. What they seem to represent is a strategy of norm evasion: adhering to the language of legality while undermining its essence. The result, unfortunately, is a norm that is still formally alive but increasingly hollowed out in its capacity to constrain behaviour.
Again, thank you for your generous reflections!