The last weeks have seen a gradual increase of pressure on the Ukrainian leadership to succumb to Russia’s imperialist demands. In particular, the United States of America under the Trump Administration have repeated their calls for a “Peace Deal” between the two states at war. The Administration suspended military and intelligence aid to Ukraine after asserting that President Zelensky was “not ready for peace”. In reaction to these developments and the public dispute in the Oval Office on February 28th, 2025, the Ukrainian President vowed to engage with the Trump Administration’s plans for peace. Military and intelligence aid has since been renewed. However, the Trump Administration remains insistent that Ukraine and Russia should terminate their hostilities and has indicated that it will put pressure on both states to reach this goal. Therefore, a “peace deal“ between Ukraine and Russia is becoming more likely.
Heading into negotiations, the US Secretary of State recently held that a transfer of territory from Ukraine to Russia will be a necessary element of such deal. Moreover, it is difficult to imagine that Russia would agree to any forms of accountability for unlawful aggression, war crimes, and other violations of the rules of armed conflict committed throughout the war. The envisioned “peace deal” is therefore set to undermine core principles of international law, notably the prohibition of aggression, territorial integrity, self-determination of peoples, and fundamental principles of international humanitarian law and human rights law.
The United Nations Security Council (UNSC) could hold a central role in affirming such flawed “peace deal” and securing its lawfulness. In its Resolution 2774 of 24 February 2025, the UNSC recalled its responsibility and duty “to peacefully settle disputes”, as indeed demanded by Art. 24 in conjunction with Art. 1, but cited Charter langue, with a significant omission: It failed to say that the Council must act “in conformity with the principles of justice and international law“ as Art. 1(1) of the Charter prescribes. If the Security Council continued along this path, it could seek to “cure” a void peace deal that otherwise would arguably be null and void (I.). However, in this endeavor, the UNSC’s powers are not unlimited, but are subject to the legal limits of the Charter (II.). In particular, the UNSC is obliged not to subvert the Purposes and Principles of Art. 1 and 2 of the UN Charter. We submit that this state of the law bars the UNSC from endorsing a transfer of territory to Russia under the current circumstances (III.). When striving to work towards both peace and justice, trade-offs will often be necessary. Yet, when balancing out the tensions arising here, the UNSC is sufficiently equipped with tools to end hostilities without notably subverting the prohibition of the use of force (IV.).
I. “Curing” the defects of a Peace Treaty
If Russia and Ukraine were to conclude a treaty involving the transfer of Ukrainian territory to Russia, it would likely be void under Art. 52 of the Vienna Convention on the Law of Treaties (VCLT). Art. 52 VCLT stipulates:
“A treaty is void if its conclusion has been procured by the threat or use of force in violation of the principles of international law embodied in the Charter of the United Nations.”
With Art. 52 VCLT, the contracting parties of the Vienna Convention (including Ukraine, Russia, and the USA) wanted to establish a legal barrier against the harvesting of the fruits of an unlawful aggression. This rule extends the Charter’s prohibition on the use of force in international relations, enshrined in its Art. 2(4), to the realm of peace treaties. Art. 52 VCLT nullifies a consent that has been “procured by” by force. This means that a causal link must exist between the threat or use of force by one state and the consent to a peace treaty by another state. Here, the degree of force employed by a state must be taken into account (Dörr/Schmalenbach, VCLT Commentary, Art. 52 para. 25). If the use of force is so overwhelming that the aggressed state is unable to choose any other option but agreeing to a treaty that seriously impairs its rights and legal status, then the link of causality is given.
Based on the recent dynamics in the negotiations, a “peace treaty” would be “procured by” the ongoing Russian aggression; the aggression would be causal for Ukraine’s consent. Although the US government recently reinstalled its military and intelligence support, it is likely that a renewed withdrawal of aid will be leveraged by the US government to promote a “deal” between Russia and Ukraine. If US military and intelligence support is withdrawn, Ukraine will be unable to uphold the current front against the aggression. Ukraine would thus only agree to this treaty because it has no other option for averting further harm in form of loss of lives, damage to infrastructure and ecologic damage. A “peace deal” transferring territory to Russia under such circumstances would therefore reward Russia for initiating and maintaining the aggression until Ukrainian resources and its international support have been close to depleted, leaving Ukraine with no other reasonable choice than to agree to a “peace deal”. This is precisely the situation which Art. 52 VCLT seeks to prohibit. Therefore, we believe that such a “peace treaty” would be void.
However, the UNSC can – as a matter of principle – translate treaty provisions in a binding decision and thus unilaterally impose them on member states. Under Art. 25 UN Charter, member states must accept and carry out the decisions taken by the UNSC. The UNSC – especially when acting under Chapter VII of the Charter ─ could endorse a “peace deal” agreed upon between Russia and Ukraine as an annex to a binding decision and thereby bestow it with binding effect upon the parties in line with Art. 25 UN Charter. Such a binding decision of the Security Council also benefits from the priority accorded to Charter obligations under Art. 103. This provision foresees that the member states’ obligations under the Charter (which includes their obligation to carry out binding Security Council decisions (Art. 25)), prevail over their conflicting treaty (and arguably also conflicting customary law-based) obligations.
Pointing to Art. 103, it has been claimed that “decisions of the Security Council under Chapter VII of the Charter, setting forth peace conditions as regards an aggressor state, are unfettered by Article 52” (Forlati 327). The prevalence of UNSC action over the rules of the VCLT is also reflected in Art. 75 VCLT which affirms the validity of a treaty forced upon an aggressor state by an enforcement measure of the UNSC. The provision does not apply directly to the case at hand, considering that Ukraine (on which the treaty is imposed) is currently being coerced and is not the aggressor in the conflict. However, it shows that the drafters of the VCLT did not want to restrict the powers of the UNSC in resolving threats to international peace. This arguably includes the conclusion or endorsement of peace treaties by the UNSC.
II. The UNSC’s obligation to act in accordance with the Purposes and Principles of the Charter
The UNSC’s competence in resolving conflicts is however limited by other factors than Art. 52 and 75 of the VCLT. The UNSC is entrusted with the “primary responsibility for the maintenance of international peace and security” (Art. 24 UN Charter). The Council needs a broad leeway for discharging its task under this broad mandate. Nevertheless, being an organ of an international organisation ─ which is as a matter of principle bound by general international law (ICJ, WHO/Egypt (Advisory Opinion) [1980] ICJ Rep 72, para 37) ─ UNSC measures are not purely political conduct that would occur in a law-free zone (ICJ, Conditions of Membership (Advisory Opinion) [1948] ICJ Rep 57, 64). Rather, the ICJ held that “neither they [the UN purposes] nor the powers conferred to effectuate them are unlimited” (ICJ, Certain Expenses (Advisory Opinion) [1962] ICJ Rep 168). As the International Criminal Tribunal for the former Yugoslavia (ICTY) held, “neither the text nor the spirit of the Charter conceives of the Security Council as legibus solutus (unbound by law).” (ICTY, Prosecutor v Dusko Tadic, Decision on the Defence Motion for Interlocutory Appeal on Jurisdiction (Appeals Chamber of 2 October 1995) ICTY-94-1-AR72 para 28). Not all contours of the legal limits of Security Council action are entirely clear. We will concentrate on the generally accepted legal parameters relevant to the current case.
First, the UNSC may not violate jus cogens norms through its actions (Conclusion 16 of the ILC’s Draft conclusions on identification and legal consequences of peremptory norms of general international law (jus cogens); ECtHR (GC) Al Dulimi v Switzerland (Judgment) [2016], para. 136; Sep. Op. E. Lauterpacht, ICJ, Bosnian Genocide, Provisional Measures (1993), para 100; ECJ, Kadi v Council and Commission [2005], paras 226 and 230). Felix Herbert has argued that a peace treaty that involves a transfer of territory is not void under Art. 53 VCLT as long as there is no “conflict” with a peremptory norm, and that such conflict is absent as long as the treaty does not purport to formally legalise an annexation. This narrow reading of Art. 53 stands in tension with the states’ and the UNSC’s obligation not to recognise a serious breach of a peremptory norm (Art. 41(2) ARSIWA; Art. 42(2) ARIO). As the ICJ recently explained, these obligations are broad and demand abstention from factual entrenchments of the prior violation (ICJ, Occupied Palestinian Territories (Advisory Opinion) [2024] ICJ Rep, e 2024, paras 278, 280 ff.). It argued that the duty of non-recognition incumbent on states and the UN vis-à-vis the Occupied Palestinian Territories extended to “any changes in the physical character or demographic composition” and “institutional structure or status of the territory“ (ibid.). Furthermore, states are required to “abstain from treaty relations”, “entering into economic or trade dealings”, “establish[ing] diplomatic mission” which validate or entrench its prolonged occupation of the Palestinian Territory” as a result of Israel’s breach of self-determination, which has jus cogens character in this context (Ibid. paras. 233, 243, 278 ff.). Considering that an aggression in violation of Art. 2(4) likewise qualifies as a violation of a peremptory norm, it is doubtful whether the UNSC could validate a treaty that cements the results of such aggression.
Further legal limits on UNSC action flow from the principles and purposes of the UN Charter itself. Art. 24(2) UN Charter explicitly says that the “Security Council shall act in accordance with the Purposes and Principles of the United Nations”. “[A]ct[ing] in accordance” with Art. 1 (“Purposes”) and Art. 2 (“Principles”) , as required by Art. 24(2), means that the Council may not, through its own actions, obstruct, undermine, or subvert those purposes and principles.
Importantly, legal limits also govern UNSC action under Chapter VII. Chapter VII measures, including the authorisation of military force, will not be measured against the inter-state prohibition of the use of force as codified in Art. 2(4) of the Charter that expressly only addresses states. Moreover, the prohibition of intervention in the domaine réservé of states does not apply to Chapter VII measures, as affirmed by Art. 2(7). Chapter VII is also apt to eclipse the customary prohibition to use military force that is often seen as generally binding the Council itself (Dörr, Use of Force, MPEPIL, para. 27).
However, the explicit mandate in Chapter VII does not mean that the UN member states generally “contracted out” of general international law by conferring enforcement powers to the Council. The proponents of the “opting out-thesis” mistakenly rely on the wording of Article 1(1) of the UN Charter. This provision indeed (implicitly) distinguishes between Chapter VII (“collective measures”) and Chapter VI action (“dispute settlement”). “International law” is mentioned only for the latter type of activity. This was a deliberate choice by the drafters. The idea was to spare the Council the necessity to inquire into the lawfulness of the involved member states’ conduct before intervening as a world “policeman” (UNCIO VI, 29 (US)). Yet, the drafters in no way sought to liberate the Council from legal limits (Peters, Art. 25 UN Charter, para. 156; UNCIO VI: p. 34 (commission I, session of 14 June 1945); p. 318 (committee 1, session of 1 June 1946); p. 23 and 24: Egypt); p. 702 (sub- committee I/1/A, 1 June 1945)). At most, an explicit and intentional deviation from ius dispositivum would be conceivable under Chapter VII, and also here only to the extent necessary to fulfil the UNSC’s mandate (Peters. Art. 25 UN Charter, para.163; ICTY, Tadic merits [1999] paras 287 and 296). Moreover, the current situation in Ukraine is an illustration of the typical factual overlap between the two types of activity mentioned in Art. 1(1) of the Charter (dispute settlement under Chapter VI and “collective measures” under Chapter VII). This overlap underlines the relevance of international law also for Chapter VII activity.
Thus, the Charter, on the one hand, explicitly allows the UNSC to authorise military action and to intervene in a member state’s affairs in the event of a threat to the peace, breach of the peace, or aggression (under Chapter VII) and to selectively disregard the prohibitions of use of force and intervention in that decision. On the other hand, this does not translate to a wholesale liberation of the Council from observing the Art. 1 purposes and Art. 2 principles. First, and foremost, such wholesale liberation would fundamentally contradict Art. 24(2)’s wording which insists on adherence to the Charter’s principles even with a view to Chapter VII powers that are mentioned in the second sentence of Art. 24(2). Second, the lex specialis of Chapter VII which forms a specific exception to the prohibitions of the use of force and intervention is first of all designed for authorising collective action. The Charter mandate to authorise military force does not comprise an allowance to endorse prior force used by an aggressor state. Third, the limitation of legal restraints on the UNSC as a “policeman” envisions a situation of urgency which calls for immediate action. An endorsement of a coerced “peace deal” would not be a measure for a single case, but would amount to UNSC legislation. Although the Council is not barred from legislating, additional formal and material limits are recognised for this type of activity (Peters, Art. 24 UN Charter, paras. 70-84). The benchmark must especially be high when the UNSC is called upon to evaluate prior conduct and pave the way for a long-term solution. While the Charter enables the UNSC to selectively depart from the prohibition of the use of force through Chapter VII measures, it does not grant the power to fundamentally subvert the prohibition of the use of force. The Council is the guardian of the principles in Art. 2 of the Charter. While it can be necessary to momentarily disregard these principles to achieve the other purposes and principles of the Charter, a long-term and impactful disregard of these principles is incompatible with this role.
Therefore, the UNSC is prohibited from adopting measures that grossly subvert a Charter purpose or a principle by producing effects that fundamentally oppose their elementary content or when the Council legitimises and inspires future violations of the Charter principles. Such action would not be “in accordance” with the Charter’s Purposes and Principles, and therefore inadmissible in terms of Art. 24(2). Such a measure would arguably transgress the legal limits of the Council’s very broad discretion and hence be ultra vires or a détournement de pouvoir. (cf ICJ, Lockerbie (Preliminary Objections) (Dissenting Opinion) [1998] ICJ Rep 110).
III. Implications for UNSC action in the current context
Endorsing a peace deal between Russia and Ukraine that entails significant transfer of territory from Ukraine to Russia would blatantly subvert the elementary (and undisputed) core of the prohibition of the use of force as addressed to Russia and would thus be ultra vires.
Art. 2(4) of the UN Charter was essentially introduced to outlaw wars of conquest to acquire territory of a different state through forcible means. Russia’s full-scale aggression since February 24th, 2022 violates the prohibition of the use of force. This also holds true for the “annexation” of four Ukrainian regions later that year. As we have detailed above, a Ukrainian concession of territory under the current circumstance would be directly caused by the aggression. A UNSC resolution which endorses such “peace deal” or – comparably to Resolution 2774 – omits any reference to international law when taking note of the transfer of territory to Russia would risk to amount to an implicit legalisation of a situation in contravention of a core element of the prohibition of the use of force.
Moreover, it would legitimise the Russian aggression. By accepting the “new realties” provoked by the aggression and endorsing a “peace deal” on this basis, the UNSC would accept the Russian approach (i.e. by using force) to renegotiating its borders as legitimate. It would reward Russia’s use of force and its perseverant war effort. This would be a blueprint for other states to acquire territory through force: initiate an aggression against a smaller state, commit abhorrent crimes in the course of the war to dissuade its population from further resisting the aggression, and then wait until the international support for the victim of aggression dwindles to the extent that the international community starts to accept the “new realities” provoked by the aggression.
Prior UNSC practice in this field (implicitly) confirms that an endorsement of a “peace treaty” involving the significant transfer of territory would amount to a fundamental subversion of the prohibition of the use of force and therefore be inconsistent with the Council’s obligation under Art 24(2) to “act in accordance” with the Charter principles. The UNSC has never (!) endorsed a transfer of territory in connection with a violation of the prohibition of the use of force. The UNSC has up to now not explained or justified its “negative” practice. Therefore, an explicit acknowledgement by the Council that its restraint in this field derives from international law is lacking. Still, the practice does demonstrate the general awareness of the Council in this regard, and also draws a clear distinction between short-term and long-term solutions. This distinction is crucial and should continue to guide UNSC action, as we will underline next.
IV. Lawful options for the UNSC
The legal limits detailed above do not hinder Council action to end hostilities between Russia and Ukraine.
First, the UNSC could endorse a ceasefire agreement that delineates the current line of contact and establishes a ceasefire, but leaves the territorial question untouched. It has already done so on numerous occasions, including an endorsement of the Lusaka Ceasefire Agreement between Uganda, Angola, Zimbabwe, Rwanda, Namibia and the Democratic Republic of Congo (DRC) through Resolution 1304 of 2000. In the Armed Activities Case, the ICJ neither questioned the validity of the agreement in terms of Art. 52 VCLT nor did it raise concerns with regard to its UNSC endorsement. Against this backdrop, Greg Fox has asked whether the ICJ would again be so deferential towards a UNSC endorsement of a peace deal between Ukraine and Russia. However, the Lusaka Ceasefire Agreement and the envisioned peace deal between Ukraine and Russia differ in one important aspect: Lusaka left the territorial integrity and the boundaries of the DRC untouched (Section 15). Importantly, it did not provide the Ugandan forces with a legal title to remain on the territory of the DRC in the aftermath. This point was highlighted by the ICJ in its 2005 judgment (para. 99 ff., 104). A treaty between Ukraine and Russia that involves a transfer of territory would therefore go well beyond the Lusaka Ceasefire Agreement in undermining Ukraine’s territorial integrity and subverting the Charter principle prohibiting the use of force.
Furthermore, the UNSC could do more than just endorsing a ceasefire agreement. In the aftermath of the Turkish invasion of Cyprus in 1964, the UNSC created a peacekeeping mission through Resolution 186 to secure the line of contact between Cyprus and Turkey. Again, Resolution 186 did not jeopardise or limit the territorial integrity of Cyprus, and the resolution explicitly referred to Art. 2(4) UN Charter.
These two examples show that the UNSC possesses a wide arrange of tools to enable and secure an end of hostilities between two states without subverting the fundamental principles of the Charter. Justice and Peace can be aligned without severely compromising one or the other. Particularly, the endorsement of the Lusaka Ceasefire Agreement shows that the UNSC can pursue peace by terminating active hostilities while remaining attached to the Charter principles. Uganda withdrew from the DRC’s territory in June of 2003 and had to answer for its transgressions of the prohibition of the use of force before the ICJ. This would probably have been different had the UNSC had explicitly authorised Uganda to maintain a military presence in the DRC or endorsed an annexation of the DRC’s territory by Uganda.
Therefore, the UNSC must refrain from doing exactly this in the context of the Russian aggression. It can endorse a ceasefire and even authorise a peacekeeping mission to secure the ceasefire. However, it may not endorse the transfer of territory from Ukraine to Russia, because this would subvert Charter principles, notably the prohibition of aggression and the protection of territorial integrity, and therefore exceed the legal limits for UNSC action.
The responsibility to keep the UNSC action within the legal limits prescribed by Art. 24(2) of the Charter principally resides with the members of the Council. As Russia and the USA are the two main advocates of a transfer of territory, it is upon the other member states, especially the non-permanent members, to block such endeavors in the Council. Moreover, France, China, and the United Kingdom are saddled with the responsibility to veto a proposed resolution.
Should the UNSC nonetheless overstep the outer legal limits of its discretion and endorse an agreement between Ukraine and Russia that entails a transfer of territory, the consequences will be manifold. This would call for a follow-up blogpost, which we sincerely hope will not be necessary.