One of the best lines in Hillary Clinton’s acceptance speech came when she quoted Jackie Kennedy’s relief that “little men” weren’t in charge during the 1962 Cuban Missile Crisis, when the world came, as Robert McNamara once told me, “just inches” from nuclear war.

If the Republican candidate for president can be “bait[ed] with a tweet,” as Clinton so memorably put it—if that disqualifies him from handling nuclear weapons, as she said—then the risk of World War III should be an issue in the 2016 campaign. Beyond his manifestly unsuitable temperament, Donald Trump will “endanger the world as we know it,” as Sen. Cory Booker put it. If you bust up the architecture of the postwar world—as Trump is threatening to do, either intentionally or out of ignorance—the war-and-peace conversation is not only fair, but necessary.

In that sense, this election may be a bit like 1964, when President Lyndon Johnson ran an ad against Republican challenger Barry Goldwater of a little girl picking a daisy while a mushroom cloud explodes in the background. It was a harsh attack, and only ran once, but it reflected deep unease about Goldwater undermining the structure of global relationships that kept the peace. “These are the stakes,” LBJ intones, as a nuclear weapon detonates.

These are the stakes this time, too. We know from history that appeasement invites war. For all his bluster, Trump has already shown himself to be an appeaser, from his “America First” slogan to his strange unwillingness to utter even a single word of criticism for Russian President and former KGB chief Vladimir Putin, one of the only major figures in the world to escape his insults (Trump, remember, even attacked the pope).

Will a President Trump allow Russia—still a serious threat to world peace even after the demise of communism—to gobble up not just Crimea and eastern Ukraine but the Baltic states? This would threaten a confrontation with Western Europe much as Adolf Hitler’s demand for “breathing room” in Czechoslovakia and other states kicked off World War II.

Clinton would stick with the world order that has served us so well. “I’m proud to stand by NATO in any threat they face, including from Russia,” she said in a little-noticed but important part of her speech.

With Trump, we’d get a dangerous secret relationship with Russian oligarchs. Three of his top advisers—Paul Manafort, Carter Page, and retired Gen. Michael Flynn—all have close ties to Russian or Russian-backed thugs and propagandists. And we now know from the FBI that Russia is intervening directly in a presidential election on Trump’s behalf by hacking the files of the Democratic National Committee on the eve of its convention.

The tilt to our old enemy is mind-blowing. Trump has been dodging questions about his views on Crimea and Ukraine and at the Republican National Convention, Trumpists quietly made a single and suspicious change in the GOP platform—they removed the plank siding with Ukraine against the territorial encroachments of Russia. Whatever Trump’s financial connections to Russian oligarchs who already favor Trump Tower apartments—and we’ll learn more soon enough—the Trump-Putin Pact is reshaping geopolitics, one tweet at time.

But even all of this is less dangerous than Trump’s attitude toward American alliances around the world. They make up the centerpiece of what keeps us all safe in our beds at night—safe not just from terrorists (intelligence sharing is critical to preventing attacks) but from the big wars it has been our immense fortune to have avoided in our adult lifetimes.

For more than 70 years, we have kept the peace between large nations through diplomacy, deterrence, and a nuclear umbrella provided by the United States that has prevented most of our allies from feeling the need to develop nuclear weapons.

Now that bipartisan commitment to collective security—strengthened by every president of the postwar era—has been repudiated by Trump, who says he would “walk away” from “outdated” NATO and allow Japan, South Korea, and other allies (not to mention adversaries) to obtain nuclear weapons.

That is why it was so important that Clinton defended the existing international order. When she spoke Thursday about being “stronger together” (a smart campaign slogan, it turns out) and dinged Trump for his dangerous “I, alone” approach, it brought to mind another “together” speech from more than 70 years ago by an American president whose presence I felt in the convention hall.

FDR’s “the only thing we have to fear is fear itself” resonated powerfully, but so did his vision for the postwar world.

In the spring of 1945, World War II—the most destructive conflict in human history—finally came to a close. It had begun a mere 20 years after the end of World War I, and Roosevelt knew that—given the development of nuclear weapons—the odds of another, even more horrendous, war were high. Working from his home in Warm Spring, Georgia, FDR prepared a speech for Jefferson Day, April 13, 1945, designed to lay out his approach to saving humanity from destruction:

“If civilization is to survive, we must cultivate the science of human relationships—the ability of all peoples, of all kinds, to live together and work together, in the same world, at peace.”

Roosevelt died the day before he delivered that speech, but the “science of human relationships” he had long envisioned—based on diplomacy (centered on his idea of an institution to be called the “United Nations”), deterrence (continued military strength at home), and a series of alliances to contain aggression—has endured.

FDR called the idea “collective security,” and it matched his idea of social security at home. It has worked magnificently. In the last 70 years, the world has experienced many wars but no big ones between large nations with nuclear weapons. The 12 presidents since FDR have all embraced collective security and understood that the American nuclear umbrella—while expensive—has kept the nuclear club small and thus millions of people alive. Asia experienced the Korean War and the Vietnam War, and Europe the Bosnian War, but those historically warlike regions have remained largely secure, prosperous, and at peace for a remarkably long time by historical standards.

In a turbulent world, the single biggest question of this election is whether we choose a candidate with deep knowledge of how to strengthen these alliances (Clinton) or one who would jeopardize the safety of us all for reasons we are only now beginning to fathom.