One can still see red poppies pinned to lapels on Remembrance Sunday across Britain. The tradition, marked this year on Nov. 10, persists even though the creators of the holiday are gone, but I doubt that younger Britons wear the symbolic flowers with the same solemnity as their parents and grandparents and great-grandparents. It’s hard to remember something that defies belief, and the further we go from the events in question, the more incredible they seem.

In the summer of 1914, in the space of mere days, Europe went from a tense peace to a catastrophic war that ultimately left millions dead, millions more gruesomely wounded, and a generation disillusioned and demoralized — “all Gods dead . . . all faiths in man shaken,” as one voice of that generation, F. Scott Fitzgerald, put it. “The blood-dimmed tide is loosed, and everywhere / The ceremony of innocence is drowned,” W.B. Yeats chimed in grimly. “We are the hollow men,” T.S. Eliot observed.

The combination of vanity and stupidity that produced the bloodbath was not cured by cynicism or resignation, however. After the war, the 1920s was a decade of nationalism, xenophobia and paranoia around the world, only to be followed by the disastrous 1930s — the decade of economic collapse and brutal totalitarianism. The resulting Second World War was even more murderous than the first, an estimate of at least 70 million dead, whole nations laid waste, economies gutted, empires ruined.

Those poppies are sold each year to benefit British servicepeople and worn in honor of their sacrifices. But for those who lived through the worst 31 years in world history — who remembered because they believed, and believed because they were witnesses to all or part of it — the flowers also seemed to represent a resolve never to make those same stupid mistakes again.

We’ve never needed the poppies more than we need them right now.

The resolve they represent was manifested in concrete ways after World War II. Led by the United States, a stable coalition of democracies agreed to link their nations and their economies through mutual defense and free trade. They created the United Nations, which, for all its very human flaws, is a forum for the exchange of words instead of bullets. They promoted peaceful development and relatively placid growth through the Marshall Plan, the World Bank and the International Monetary Fund.

Nobody better represented this resolve than a pair of Midwestern politicians born in the spring of 1884. Harry S. Truman was a Democrat from western Missouri and a veteran of World War I. Arthur Vandenberg was a Republican from western Michigan and a fierce critic of the long-serving president from Truman’s party, Franklin D. Roosevelt. For 10 years, from 1935 to 1945, the two men served together in the Senate, where they typically found themselves on opposite sides of important issues.

When Roosevelt died, Truman, who had become vice president just 82 days earlier, was faced with the formidable challenge of learning and applying the lessons of catastrophe. What allowed him to succeed was that Vandenberg had learned many of the same lessons. As chairman of the Senate Foreign Relations Committee, Vandenberg declared that “politics stops at the water’s edge” and shed his nationalism and isolationism to embrace Truman’s vision of an international order. The decades of relative peace and prosperity that followed stand among the great bipartisan achievements of U.S. history.

Their achievement is unraveling. In part, it’s coming apart because for the first time since the World Wars, the United States has a president who lacks the resolve of the poppies. Donald Trump’s impulses are nationalist and isolationist. He perceives U.S. leadership of allied democracies as a mug’s game in which freeloading countries rob Uncle Sam blind. In a candid, blunt interview with the Economist, French President Emmanuel Macron diagnosed the resulting crisis. Europe stands at “the edge of a precipice,” he said, abandoned by the United States and beginning to split along ancient fault lines.

But Trump is not the only one who seems to believe that the world can safely tolerate a collapse of order. The poppy-wearing Brits are drowning under a wave of resurgent nationalism, with Parliament in crisis over withdrawal from the European Union.

Nationalism is ascendant across Europe, from Poland to Hungary to Turkey. The dormant pathologies of xenophobia and anti-Semitism are awakening throughout the West. Fascists have marched in Charlottesville and in Marseille and even, incredibly, in Berlin. Stalinism is countenanced in the outlaw state of Vladimir Putin’s Russia. China is building a 21st-century totalitarianism.

Bankrupt impulses and failed ideologies of the 1920s and 1930s are taking root again in another generation of disillusioned and demoralized leadership. But it’s not too late to remember the bitter, bloody fruit of those fields and change course. What better time to start remembering than on Remembrance Day?

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