As the bloodiest century in human history drew to a close, Americans looked back at the catastrophic mistakes that paved the way for World War II and the Cold War.

Chief among them: the 1938 Munich agreement, British Prime Minister Neville Chamberlain’s misguided effort to appease Adolf Hitler, and the disastrous 1945 Yalta agreement that partitioned Europe and left the world teetering on the brink of nuclear Armageddon.

At the start of a hopeful new century came a new axiom of U.S. foreign policy: No more Munichs, no more Yaltas.

To which we must add: No more Budapests.

On Dec. 5, the world will mark the 30th anniversary of the Budapest Memorandum of Security Assurances, the disastrous agreement that paved the way to today’s bloody war in Ukraine. As President-elect Donald Trump prepares to negotiate an end to that struggle, he should heed the lessons of Budapest — so he does not repeat them.

After the Soviet Union collapsed in 1991, Ukraine inherited nearly 2,000 nuclear weapons, along with intercontinental ballistic missiles and strategic bombers to deliver them — making it the world’s third-largest nuclear power. So, in 1994, President Bill Clinton brokered an agreement among Russia, Ukraine, the United States and Britain in which Ukraine agreed to give up those weapons. In exchange, Russia pledged to “respect the independence and sovereignty and the existing borders of Ukraine” and to “refrain from the threat or use of force against the territorial integrity or political independence of Ukraine.” Moscow also agreed to refrain from the “threat of aggression in which nuclear weapons are used.” And the United States and Britain gave Ukraine security guarantees, promising “to provide assistance to Ukraine … if Ukraine should become a victim of an act of aggression.”

Those guarantees proved empty. In 2014, Russian President Vladimir Putin invaded Ukraine and annexed Crimea, in direct violation of the pledges Russia made in Budapest. In the face of this aggression, President Barack Obama’s administration failed to hold up the United States’ end of the bargain, refusing to provide Ukraine with weapons to defend itself.

Instead, out of fear that arming Ukraine would provoke Moscow, Obama offered Ukraine only nonlethal aid.

When Trump took office, he reversed Obama’s policy of appeasement and became the first president to provide Kyiv with lethal aid. Trump also got tough on Putin in a host of other ways — imposing crippling sanctions on Moscow, expelling Russian diplomats, launching a cyberattack on Russia targeting St. Petersburg’s Internet Research Agency, giving the green light for the U.S. military to take out hundreds of Wagner Group mercenaries in eastern Syria and persuading NATO members to increase their defense spending by $400 billion. Result? Putin paused his aggression during Trump’s term.

But after Joe Biden took office, Putin resumed his conquest of Ukraine. In the wake of Putin’s 2022 full-scale invasion, Clinton acknowledged that the Budapest agreement he negotiated was to blame. “I feel a personal stake because I got them to agree to give up their nuclear weapons,” Clinton told Irish broadcaster RTÉ. “None of them believe that Russia would have pulled this stunt if Ukraine still had their weapons.”

What lessons can we learn from the failure of the Budapest accord?

First, Putin does not want peace; he wants Ukraine. He will violate any international agreement Russia signs to achieve his objective. The only way to stop him is to make his objective impossible to achieve. As we have seen, Putin believes he can wait out a strong U.S. president until another weak one replaces him.

If allowed to do so, he simply will use a cessation of hostilities to pause, reconstitute his forces and resume his invasion when the time is right. If Trump wants a peace that outlasts his presidency, that agreement must create conditions that make a resumption of war impossible.

Second, Western security guarantees are worthless unless they are backed up with Western military might. A lasting peace will require that whatever agreement Trump negotiates creates defensible borders, with a demilitarized zone enforced by an international peacekeeping force (made up of European, not U.S., troops).

Putin must understand that this agreement is final, and that if he ever tries to resume his invasion, he will not be fighting just Ukraine.

Third, Ukraine must be militarily strong enough to deter Russia. Giving up its nuclear deterrent, and depending on others to protect it, was a mistake. Ukraine will need to create a conventional deterrent so powerful that Russia will never take it on. This means that, even if Trump succeeds in forging peace, the imperative to arm Ukraine will continue. We must find mechanisms to increase the flow of U.S. weapons headed to Kyiv that do not require U.S. taxpayers to bear the cost.

If Trump wants to avoid presiding over a historic failure like Budapest, he needs to avoid the trap of trying to appease Putin with promises of Ukrainian neutrality and disarmament. He does not want to join Neville Chamberlain in the pantheon of leaders who promised peace in our time but delivered the opposite.

Trump says he wants to prevent World War III. If that is the case, he should do what he did in his first term and secure peace through strength.

Marc Thiessen writes a column for The Washing Post on foreign and domestic policy.