Lately, I can’t shake the image of a young man on a battlefield in France or the South Pacific. It’s 1944. He’s dying — one more incremental death amid the worst carnage the world has ever seen. What if I told you that experts’ estimates of the death toll in the Second World War range from 50 million to 85 million?

Would you skim right by, or would you pause to consider what hellish conditions would create a margin of error of 35 million lives? Nearly the entire population of California — gone, or never there to begin with. As Erich Maria Remarque wrote in his novel The Black Obelisk, “one dead man is death — and two million are only a statistic.” And 50 million, or 85 million, is a chillingly vague statistic indeed.

So I’m thinking of one dying soldier rather than mind-boggling millions. He could be any one of the lost multitude, but let’s give him an identity: Second Lieutenant Jack Lundberg from Woods Cross, Utah. On the eve of D-Day, with a premonition that he would never see the Great Salt Lake again, Lundberg wrote to his parents about his love for them, his gratitude for life and his willingness to sacrifice all for his country.

“I feel that in some small way I am helping to bring this wasteful war to a conclusion,” he said.

I’ve been wondering how I could explain to such a man that many of his fellow Americans, most notably the President, have already forgotten where his war came from and why he had to fight it. America in the age of Trump is undermining, if not dismantling, the international framework put in place to prevent such a catastrophe from happening again.

At 25, Lundberg was no stranger to America First, protective tariffs and nationalism. No American of his age or older could be. These themes had been among the most prominent topics for public debate throughout his short life. And each had contributed, in one way or another, to the chain of events that took Lundberg to war.

The isolationism that fuelled the original America First movement died with the first bomb at Pearl Harbour. The Smoot-Hawley tariffs of 1930 had deepened the Great Depression, and that crisis fanned nationalism from Berlin to Tokyo.

The danger and folly of these policies was written in an ocean of blood — Lundberg’s and all the others’.

So when the wasteful war finally ended, the United States led the world away from those policies and built institutions to prevent new eruptions. The United Nations was formed. The World Bank and International Monetary Fund were created. The General Agreement on Tariffs and Trade was negotiated — and later replaced by the World Trade Organisation. The seeds of the European Union were planted, and America’s commitment to stability was made concrete through the North Atlantic Treaty Organisation.

No snivelling Eastern elitist erected this framework. It was a bipartisan project guided by a Missouri farm boy, Harry S. Truman. A chastened former isolationist, Arthur Vandenberg, the senator from Michigan, delivered Republican support to the Democratic president.

“Politics,” the senator declared refreshingly, “stops at the water’s edge.”

Because these were human institutions, not one of them has been perfect. By any historical measure, however, the postwar order has been a tremendous success, for the United States as well as its allies.

This American-led network of international institutions has produced the longest period without a war between great powers since the days of the Roman Empire. We’re at 73 years and counting. Before its creation, Europe had plunged the world into two global wars in the span of only 25 years. This alone, peace among the great powers, has been worth every penny spent and every hour of haggling.

But peace is not the only benefit. There is prosperity, too.

In 1945, the American economy had its best year to that point, producing $228 billion in gross domestic product. Adjusted for inflation, that is $3.2 trillion in today’s dollars.

The people working round-the-clock in America’s factories and dawn to dusk on its farms, straining to feed, clothe and arm the Allied war effort, might have imagined that no economy could ever run faster. And yet, during the ensuing decades of peace, the GDP of the United States has grown to roughly $20 trillion — more than 500 per cent. That has been accomplished while also enabling the ruined nations of Europe and Asia, partners in free trade, to achieve similar economic miracles.

Warts and all, this Pax Americana is the unparalleled gem of diplomatic history and the epitome of bipartisan achievement. Barack Obama was widely seen as backing away from America’s lead role; now Donald Trump is reviving the very policies that once darkened the world.

I cannot shake the image of that young man. He is asking: How can you forget?

•David Von Drehle writes a twice-weekly column for The Washington Post. He was previously an editor-at-large for Time Magazine, and is the author of four books, including Rise to Greatness: Abraham Lincoln and America’s Most Perilous Year and Triangle: The Fire That Changed America