In an essay in The New York Review of Books, Jessica Mathews points out that since 1945, Americans of both political parties have accepted three principles. First, that America’s security is enhanced by its broad and deep alliances around the world. Second, that an open global economy is not a zero-sum game but rather allows America to prosper and others to grow. And finally, while there was debate about whether dictatorships were to be “tolerated, managed, or confronted,” in the end there was a faith in democracy and its advantages. Mathews notes that for 30 years now, Donald Trump has attacked these views as costly naivete that has allowed the world to rip off America.

Given the magnitude of the policy shift, it is worth recalling why America adopted this outward-looking approach in the first place. It all started with Franklin Roosevelt, as Nigel Hamilton explains in his superb book “Commander in Chief.” By 1943, while victory was still a distant prospect, Roosevelt began to imagine a postwar international system. Hamilton brilliantly sets out Roosevelt’s foresight, determination and skill in establishing a new world order.

Neither of FDR’s key wartime allies was much interested in his approach. Josef Stalin, a communist autocrat, would resist many of his ideas, and Winston Churchill was stubbornly committed to continuing Britain’s rule over its vast empire. Roosevelt wanted something different: to establish an enduring peace in which freedom could flourish. That meant the unconditional surrender of Germany and Japan, to wipe the slate clean of fascism and militarism. And it meant that Britain and France would have to decolonize Asia and Africa. Roosevelt despised the system of colonial exploitation, and he believed that ultimately it created the conditions that led to revolution and war. He also wanted open trade, rather than the ruinous protectionism of the 1930s. To secure all this, FDR understood that America would need to be permanently engaged with the world in a way it had never been before.

Hamilton vividly describes how, in the midst of directing military strategy in the largest conflict in human history, Roosevelt always kept his eye on postwar planning. With Congress and the public still suspicious of American involvement, he juggled various plans and proposals to make sure that this time, unlike after World War I, America would help keep the peace. He needed Churchill’s and Stalin’s support, which is why he kept trekking around the globe to meet them at summits. (To understand the strain on FDR, keep in mind that Roosevelt’s trip to meet Churchill at Casablanca in 1943 entailed a long train ride to Miami, a 10-hour flight to Trinidad, a nine-hour flight to Brazil, a 19-hour flight to Gambia, and finally a nine-hour flight to Casablanca. All this for a man who was paralyzed, had a failing heart, and had not taken a plane since 1932.)

Roosevelt’s vision for a global system did not work exactly as he had hoped, chiefly because of the Soviet Union and its postwar behavior. But much of it did happen, from the United Nations to an open global trading system to the decolonization of Europe’s empires. And the great holdout to America’s vision, the Soviet Union, itself collapsed in 1991.

The results have been astonishing. Many historians have pointed out that we live in unprecedented times. The period since 1945 has been marked by the absence of war between the world’s major powers. Most of prior human history is a tale of economic mercantilism, political conflict and repeated war. Since 1945, we have lived in what John Lewis Gaddis called, “the Long Peace.” Through the Long Peace we have also had decades of rising incomes, living standards and health throughout the world, including in the United States.

When Roosevelt was beginning to design his system, he was the dissenter. The dominant foreign policy ideas in America at the time were represented by a movement called “America First.” Nativist, isolationist and anti-Semitic, the movement held that an outward-oriented America was a policy for suckers. It took Hitler and World War II to make Americans recognize that, for a country of America’s size and scale, isolation and narrow self-interest would lead to global insecurity and disaster. One wonders what it will take to make today’s America Firsters relearn that same lesson.