Rules of International Law after the war: The Ana­tomy of Peace

At this year's UN General Assembly in September 2025, the United States presented two new peace plans – for Ukraine and Gaza. In New York, Trump presented a 21-point plan for Gaza to several Arab heads of state and government, with the aim of establishing a ‘lasting peace order’. The 21 points have since been reduced to 20. According to media reports, the previous plan also included Israel's agreement not to carry out any attacks in Qatar in the future. Benjamin Netanyahu has already agreed to the 20 points, but Hamas has yet to give its approval. In addition, the international community – without the United States and Israel – discussed a two-state solution, set out in the so-called ‘New York Declaration’.

Framed as a peace plan, the United States also announced vague promises for a peace settlement in Ukraine, wavering between full support for Ukrainian sovereignty and possible territorial concessions.

Such peace plans operate in the open space of the post-war order, where legal norms are not fixed but negotiated. This process can itself become a source of conflict, because negotiated agreements set normative benchmarks for future coexistence and interaction between the parties involved. These agreements have a decisive influence on what the fragile post-war period will look like. International law does not regulate this transitional phase after violence with clear rules. Rather, political power relations shape this period. In contrast to ius ad bellum (law in peacetime) and ius in bello (law that applies in wartime), ius post bellum – post-conflict law – is an open flank of international law. It is a void that is filled with very different content depending on the balance of power. The pressing question posed is: What does legally apply after war? 

Peace plan, ceasefire, peace treaty – three instruments, three legal effects

A war can be suspended or settled in various ways. A distinction is made between a peace plan, a ceasefire agreement and a peace treaty.

A peace plan is usually a political proposal or framework intended to pave the way for negotiations. As a so-called soft-law instrument, it is not legally binding. Its weight depends on the standing of its initiators and its acceptance by the parties involved in the conflict. 

In contrast, an armistice is an agreement binding under international law to temporarily cease hostilities. It is based on international humanitarian law, in particular the Hague Convention (Articles 36-38, 40) and the Geneva Conventions (for example, Article 109 of GC III contains rules on the exchange of prisoners ‘after the end of hostilities’). Typical contents include humanitarian provisions (food and medical supplies), disengagement (demilitarisation and disarmament) and prisoner exchanges. However, its effect is limited: the state of war does not end, the conflict remains latent, and ius in bello continues to apply. In practice, a ceasefire often proves to be a fragile truce.

A peace treaty goes significantly further than this. It formally ends the state of war and establishes a new legal order. It creates rights and obligations, transforms violent relationships into normative relationships, integrates institutional mechanisms for dispute resolution, contains security guarantees and regulates territorial issues. Transitional provisions and monitoring and sanction mechanisms are also often providedto ensure compliance. Classic examples of peace treaties are the Camp David Accords between Egypt and Israel in 1978 and the Two Plus Four Treaty between the Federal Republic of Germany and the German Democratic Republic with the victorious powers of the Second World War. In contrast, the Oslo Accords of 1993 and 1995 between Israel and the Palestinian Liberation Organization (PLO) are not considered classic peace treaties, but rather interim agreements within an ongoing peace negotiation process.

Limits of peace treaty law and imbalances

However, negotiated post-war arrangements are often susceptible to asymmetrical enforcement of interests. More powerful actors (such as Russia or the United States) find it easier to assert their interests due to their position and can shape rules in such a way that they primarily benefit them. This can take the form of non-binding agreements (soft law tendencies), bilateral side agreements or external (extraterritorial) influence. The consequences are often renewed tensions or escalations of violence. 

There also remains a significant risk that contract clauses disproportionately favoring one party could be secured through means violating international law. However, according to Article 52 of the Vienna Convention on the Law of Treaties (VCLT), a treaty is void if its conclusion is the result of coercion (through threats or force). The decisive criteria is whether there is a substantive causal link between the coercive measure and the conclusion of the treaty. According to Article 69(1) VCLT, such a treaty is void from the outset and, according to Article 44(5) VCLT, cannot be divided into a valid and a void part. Only obligations that arise from general international law remain valid, such as the obligation to end the unlawful use of force, humanitarian access or prisoner exchanges.

For Ukraine, this means that a peace treaty involving territorial concessions under Article 52 of the Vienna Convention on the Law of War could be null and void as long as Russian troops continue to illegally occupy parts of Ukraine – and as long as Ukraine is not involved in the negotiations. However, an agreement aimed at the complete restoration of Ukraine's territory does not fall under Article 52 of the Vienna Convention on the Law of War, as Russia would not be benefiting from its use of force. 

Similar considerations apply to Gaza. Treaty provisions on territorial reorganization, forced population movements or permanent foreign administration without free consent would be null and void under Article 52 of the Vienna Convention on the Law of Treaties and the restrictions of peremptory international law (ius cogens) under Article 53 VCLT. On the other hand, agreements on ceasefires, humanitarian corridors, prisoner exchanges or hostage releases would be legally viable. 

Quick peace vs. protection of free consent

There is a tension between the legitimate goal of fast peace and the protection of free consent that needs to be balanced. To ensure that the peace treaty does not become a tool for the victors and perpetuate the existing power relations of a conflict, an additional framework under international law is needed to secure lasting peace through egalitarian minimum standards.

A ius post bellum could set guidelines for the post-war order. Instead of bilateralism, there is a need for multilateral anchoring and inclusive negotiation processes that involve civil society, women and minorities. The involvement of those affected creates legitimacy (local ownership), and the participation of women in peace processes has been shown to promise greater stability. This brings in perspectives that would otherwise be overlooked. 

Transparency and oversight through independent review mechanisms are also important components. International mandates should not be permanent, but rather transitional solutions. They can, for example, be subject to sunset clauses, in other words provisions stipulating that a mandate expires after a certain period of time. Another option is to have clear exit strategies like well-defined agreements on how and under what conditions an international mission or administration should be terminated – in other words, an exit plan to ensure that responsibility is handed back to local actors in an orderly manner. This can prevent deadlocked foreign administration (e.g. UNMIK in Kosovo from 1999 to the present day or MONUSCO in Congo from 1999 to the present day). Agreed obligations should not only be sanctioned bilaterally, but also reviewed and processed in multilateral bodies to ensure objectivity and avoid arbitrariness.

Such a ius post bellum would not only complement peace treaties, but also provide a necessary framework to prevent treaties from degenerating into mere fig leaves for hegemonic power. It would bind power politics to norms, procedures and institutions committed to the goal of sustainable peace consolidation.

War as a cultural ‘constant’? 

In his book The Culture of War, John Keegan developed the thesis that war is part of our civilisation – so deeply rooted that it can hardly ever be completely overcome. War, as a cultural practice, is deeply interwoven with human existence. These theses, thoroughly explained in the book, are provocative and strike a nerve. Must war be accepted as a cultural constant despite the normative order of peace? 

If war is a lived culture, then peace can also be a culture. Peace is a conscious decision and a process of reorganization. It can be shaped legally, politically and culturally and must be deeply rooted in society in order to have a lasting effect. The 2025 Peace Report shows that social inequality, polarization and the formation of opposing groups are driving factors for social division. They reinforce feelings of being disadvantaged and fear, which in turn can increasingly turn into violence. Post-war periods are particularly sensitive and vulnerable in social terms. However, egalitarian minimum standards can prepare the ground for a growing culture of peace. 

The future, says Florence Gaub, is everything we can imagine it to be. Then it is up to us to shape it – with justice, courage and the conviction that peace is not only possible, but mandatory.

This text is a translated and slightly revised version of the article published in German on September 30, 2025.

Lisa Wiese is a fully qualified lawyer and doctoral candidate at the Chair of European Law, International Law, and Public Law at Leipzig University.