The White House suggests that it is in the final stages of pushing for a Ukraine-Russia peace deal. And there are modest signs of progress, including a U.S.-Ukrainian agreement on mineral rights and reconstruction, as well as a meeting between U.S. President Donald Trump and Ukrainian President Volodymyr Zelensky in Rome that did not devolve into a shouting match.

But even assuming that Ukraine succumbs to U.S. pressure and reaches an agreement with Russia, all three states face a fundamental problem: International law will regard any peace treaty procured by force like Russia has deployed since 2014 as void. The remedy for this—U.N. Security Council approval of the agreement—warrants that institution playing a role sooner rather than later in the conflict’s outcome.

The Vienna Convention on the Law of Treaties went into force in 1980 after three decades of drafting and ratification. Today, 116 states are bound by its terms, including Russia and Ukraine. The United States, while not a party, accepts most of its provisions as customary international law. Here’s the catch for Ukraine and Russia: Article 52 of the convention states, “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.”

Prior to World War II, international law allowed states to go to war and coerce treaties as a result. The U.N. Charter, drafted under U.S. leadership, changed things fundamentally. It prohibits both the threat and the use of force, except in cases of self-defense or with U.N. Security Council authorization. The idea was to avoid future agreements like the infamous 1938 Munich Agreement where Britain, France, and Italy agreed to Nazi Germany’s annexation of Czechoslovakia’s Sudetenland. Thus, the experts who produced the first draft of the Vienna Convention saw what became Article 52 in simple and categorical terms: All treaties procured by threats or use of force breaching the charter are not simply voidable but void. The International Court of Justice has described Article 52 as customary international law, making it binding on all states.

Although Russian President Vladimir Putin, and more recently Trump, have suggested that Ukraine bears responsibility for the current conflict, that premise is simply untenable. Whether the conflict to be resolved started in 2014 with “green men” in Crimea or the full invasion of Ukraine in February 2022, Russia is in violation of the U.N. Charter. It has no viable claims to self-defense or Security Council approval. Indeed, the U.N. General Assembly concluded as much in March 2022. Even if Ukraine agreed to cede territories in an attempt to negotiate at least some favorable peace treaty provisions (such as security guarantees and the recognition of its right to pursue EU membership), that cannot salvage things. Any peace agreement that makes territorial or financial concessions to Russian interests must be viewed as procured by the force Russia has deployed, which makes it invalid.

Fans of realpolitik may wonder why the invalidity of a Ukrainian-Russian peace treaty matters so long as the parties accept it politically. And certainly, states increasingly rely on nonbinding agreements to deliberately skirt their rights and obligations under international law. The so-called Iran nuclear deal is a paradigmatic example: By not making it a formal treaty, the Obama administration avoided a hostile U.S. Senate approval process, which then allowed Trump’s first administration to walk away from the deal without triggering the right to any legal remedies for Iran and other participants. That deal’s very impermanence, however, highlights the risks of such an approach.

The treaty form has endured for millennia for a reason. Putting promises into a treaty signals to the parties and others a serious commitment that is absent in more ephemeral promises. If a Ukraine-Russia peace treaty is void before the ink is dry, neither side will have the legal ability to demand that the other conform to its terms. Neither can deploy sanctions or other countermeasures if the invalid treaty is breached. Instead of peace, this will create a dangerous instability; at any future point, Ukraine or Russia could simply claim that the deal was always void, creating ample room for opportunistic violations of its terms if either state sees an advantage in restarting the conflict. Third-party countries would also be free to decline to recognize the deal or to facilitate its observance.

There are two ways to avoid an invalid treaty. The first is based on self-defense. If the original victim of aggression repels the invasion and dictates terms to the original invader, that treaty is valid even though it is coerced. That possibility seems unlikely in Ukraine at present.

The other option is to allow the U.N. Security Council to ratify the deal under its authority to maintain international peace and security. It has done so in the past, including when it ended the war in Kosovo in 1999 and the fighting between Ethiopia and Eritrea in 2000. It could do so again here. That would mean the United States cannot go it alone in hammering out a peace deal. Instead, it needs to bring along a majority of Security Council members, including those who, like the United States and Russia, have a veto—namely, Britain, France, and China. Bringing in these countries will, however, likely mean accommodating additional peace terms that will be less favorable to Russia, strengthen Ukraine’s position, and give a number of key European voices their desired seats at the table.

The White House will no doubt want to ignore or resist the Vienna Convention’s rule voiding treaties generated by coercion. Trump’s second term is an apparent exercise in casting aside long-standing international commitments and institutions; it would be surprising if he did not do so here. But the obligations that the U.N. Charter and the law of treaties impose are not subject to the whim of a hegemon. The rule voiding coerced treaties exists precisely because all U.N. member states accepted a rule-of-law framework to avoid a world where might is always right.

U.S. Secretary of State Marco Rubio must recognize the problem coercion will pose for any future peace treaty between Russia and Ukraine, no matter the pressure the United States imposes to achieve it. He should aim to convince the White House that cooperation from the Security Council—now—is the best path forward. Greece holds the Security Council presidency for all of May and is thus well positioned to place the item on the council’s agenda. Involving the Security Council can, for one, help ensure a lasting peace between Ukraine and Russia that all countries will respect. But just as importantly, it can ensure that the result is a just peace—a deal the Security Council can certify comports with international law. At a time of rapid global change and increasing disorder, it is important that states signal that the rule of law—not aggressive wars—will continue, however imperfectly, to serve as the global order’s defining framework.