Critics of President Donald Trump's foreign policy seem to think his contempt for the liberal world order is something alien to modern American politics. It's not.
And until Trump's critics understand the popular appeal and deep roots of this contempt, it will be hard to restore an unraveling international system.
The theory is that since 1945, U.S. political leaders more or less supported America as the anchor of a global order that promoted free trade, deterred military aggression and encouraged open societies. Then Trump won the 2016 election and everything changed.
As Robert Kagan shows in his new book, "The Jungle Grows Back," this is only partly true. Between 1945 and 2008 U.S. presidents of both parties favored a global military presence and free trade.
This was the American-led world order envisioned by George Kennan and Dean Acheson.
But there has always been a strain of American politics that favored retreat from the world. In the first years of the Cold War, some Republicans questioned the wisdom of rebuilding Europe's economies with the Marshall Plan. Following the Vietnam War, the left and the center-right began to question the wisdom of containing communism. No less a cold warrior than Richard Nixon mused that the world would be safer if Europe, America, China, Japan and the Soviet Union were equally powerful in relative balance.
In 2000, George W. Bush campaigned against the U.S. getting bogged down in nation building, an indirect rebuke of his father's vision of a "new world order" after the collapse of the Soviet Union.
Not until Barack Obama, however, did a U.S. president begin to challenge the bargain America had struck with the world after 1945. As Kagan sees it, Obama "set out to reposition the United States in a more modest role appropriate to a new era of global convergence."
This meant seeking rapprochement with Cold War rivals like Cuba, Iran and Russia. It meant withdrawing U.S. troops from Iraq, if only temporarily (some of them went back after the Islamic State's rampage in 2014).
Kagan is careful to point out that Republicans in Congress enabled Obama's efforts to remake U.S. foreign policy into something less exceptional. They skewered him for providing air support for the NATO mission to stop Muammar Qaddafi's massacre in Libya. They opposed his efforts to punish Bashar al-Assad's use of chemical weapons in Syria.
Both Obama and his Republican critics were correctly reading the mood of their constituents. Kagan observes: "And so beginning two years into his presidency, and even as the liberal world order began to show further signs of strain and cracking around the edges, Obama did what the American people evidently wanted, which was very little."
This is why Kagan's book is so important now. In clear and forceful language, it makes the case for America continuing its role as the guarantor of a liberal world order. Without a powerful liberal democracy as the anchor of that system, the world that gave rise to European and Japanese fascism will return.
Other great powers will seek to dominate their weaker neighbors. Accepting the world as it is requires accepting a world in which war is more likely.
Kagan illustrates this point by asking critics of American interventionism to consider the last quarter of a century. Despite terrorist attacks, the war in Iraq, the atrocities in Syria and the migration crisis in Europe, the last 25 years "have been characterized by great-power peace, a rising global GDP, and widespread democracy," he writes.
This compares favorably to the first 45 years of the 20th century, which saw two world wars, the rise of communism and fascism, the Holocaust and the Ukrainian famine.
This relative prosperity, peace and freedom did not happen by accident. It happened because of U.S. military, economic and diplomatic power.
An American-led liberal world order, Kagan reminds us, is not the natural order of things. It is not normal, to echo the refrain of the Trump resistance.
For most of human history, the strong have dominated the weak and authoritarians have prospered. It is up to Americans at this hinge in history to convince their fellow citizens that a retreat from global leadership endangers us all.