Donald Trump has made “America First” his clarion call. That phrase was the centerpiece of his campaign, and he doubled down on it in his inaugural address, declaring, “From this day forward, it’s going to be only America First.”

As Trump is well aware, America First was the name of a notorious organization that crusaded for America’s isolationism in World War II. In his channeling of that group, our new president aims to turn the United States into Fortress America, closing its borders, walling it off from the rest of the world, and focusing entirely on its own self-interest — as he defines it.

The U.S., of course, was hardly the only nation to put its own interest first in the years before World War II. For the sake of peace — its own peace — Great Britain stood quietly by as Hitler began his march to war. So did France. Smaller European countries, like Norway, Belgium, and the Netherlands, also refused to get involved, relying on their neutrality to keep them safe. Catastrophe was the result, as one by one, Hitler picked these countries off, leaving Britain as the only European nation holding out against the Nazis.

Belatedly, Britain and Europe did join forces during the war, thanks to Prime Minister Winston Churchill, who invited the leaders and military forces of seven occupied European nations to take refuge in London in the summer of 1940. Together, the British and Europeans formed a partnership that ended up making a significant contribution to the Allied victory.

During their long stay in London, the exiled Europeans worked and socialized together in ways that would never have been possible before. The horror of their countries’ invasion and occupation had disabused them of the belief in every country for itself, and they began to explore the possibility of banding together to prevent such a conflagration from happening again. Those discussions planted the seeds for the postwar campaign for European unification — an extraordinary effort that helped lead to more than half a century of peace and prosperity for western Europe.

In a stunning reversal from its prewar isolationism, the United States went to great lengths in the late 1940s to help Europe achieve military and economic security. As a founding partner of NATO, we became a permanent force in maintaining peace in Europe; through the multi-billion-dollar Marshall Plan, we helped jump-start the economic recovery of a war-devastated continent.

If Trump had been around then, he would have labeled President Truman and his administration as fools for squandering America’s wealth on foreigners. In Trump’s view, foreign aid and alliances are a zero-sum game: they win, we lose. In fact, such military and economic support was indisputably in America’s self interest: by helping to stabilize Europe and other allies, our own security and economy were strengthened as well.

Now, under Trump, we are going back in time, “embarking,” as the conservative columnist Charles Krauthammer has noted, “upon insularity and smallness.” Although I usually don’t agree with Krauthammer’s views, I think he’s spot on when he says, “Global leadership is what made America great. We abandon it at our peril.”