By the end of the last century, even as Communism, at least in Europe, had ceased to be an existential threat, the dependence of democratic allies on the United States began to cause greater strains. Mr. Trump is not the first president to voice his resentment about Europeans and East Asians taking American security too much for granted and not paying their fair share; Barack Obama said so, too. Only, Mr. Trump’s resentments are expressed with more vulgarity and less knowledge.

The Japanese and Europeans pay plenty. But the system of dependence does sometimes make America’s allies behave like adolescents: quick to criticize and moralize about America’s policies, and equally quick to seek its protection in a crisis. It was the Americans, not the Europeans, who forced a halt to the war in Bosnia in 1995, a conflict that brought back concentration camps on European soil for the first time since Hitler’s Third Reich.

Pax Americana is now faced with a dilemma that European empires had to contend with before. Even as it becomes clear that nations should be weaned from foreign hegemony, the transition is almost always messy and sometimes bloody. The argument of imperialists has always been that the peoples, or nations, they rule are not yet ready to rule themselves. Wouldn’t chaos come from the hegemon’s withdrawal? Harold Macmillan, the prime minister of Britain when many African nations ended British imperial rule, had an answer. People are never ready to rule themselves, he once said, quoting an Africa specialist in his Foreign Office, but the longer an empire rules and the more talented rebels are locked up in prison, the harder it will be for people to get learn how to govern once independence comes.

Pax Americana was never formally an empire. And the allied nations in Europe and East Asia are not colonial possessions. But the level of those states’ dependence is a problem, especially at a time when an erratic, spiteful and isolationist president is in charge of the United States. The Europeans should be taking more responsibility for their own security. The Japanese should have a national debate about their Constitution, written by Americans in 1946, which bans their participation in any combat outside their borders. Otherwise, the demeaning state of national adolescence among American allies will go on, inflaming resentments all around.

A slow and orderly transition from an increasingly tattered Pax Americana is needed. But the great danger of the Trump presidency is that the transition might take place in an atmosphere of chaos and panic. This is why the betrayal of the Kurds could have such serious costs. If the bona fides of the dominant partner in an alliance can no longer be trusted, the partnership will disintegrate fast, with many unintended consequences.

The current lack of confidence in the United States and in what has become of the order it created is already apparent in Europe. Angela Merkel, the German chancellor, stated after a NATO meeting in 2017 that Europeans could no longer completely rely on Britain and the United States and so Europeans should be prepared “to really take our fate into our own hands.”

This will be hard enough in Europe, where the European Union still has no common foreign policy, let alone a unified defense force. The Japanese, with their constitutional problem and lack of any formal alliance apart from the security treaty with the United States, are in an even worse situation. Terrified of China’s rising power in Asia, which represents a new and far more oppressive hegemony, the Japanese still have to rely on America when it no longer is reliable, no matter how many rounds of golf Prime Minister Shinzo Abe plays with Mr. Trump.

For a panicky Japan, a frightened South Korea, a bellicose North Korea, a malevolent Russia and a powerful Chinese dictatorship seething with resentful chauvinism, the unraveling of Pax Americana could result in violent conflict. And where careful diplomacy is needed to replace complete dependence with more equal partnerships, Mr. Trump is more inclined to wield a wrecking ball.

Apart from the risk of war, Mr. Trump’s posturing is having another serious consequence. The strength of the United States never relied only on its often-misguided use of military power. American democracy, with all its flaws, was a model, even an ideal, for much of the world. Refugees from tyranny and war continued to see the United States as a haven. Popular American presidents such as John F. Kennedy and Barack Obama were idolized for that reason.

This has come to an end. Mr. Trump is no model of democracy or American generosity. On the contrary, he is a model for strongmen all over the world who view democratic checks and human rights with contempt. In the past, such autocrats would at least have had to contend with American censure — with the notable exception of some of the brutes on our side during the Cold War, like Suharto in Indonesia or Gen. Augusto Pinochet in Chile (all foreign policy carries its own hypocrisies). They could hold on to power in their own countries, but they could never win the esteem of the world public opinion.

Now that Mr. Trump has given his stamp of approval to autocrats and despots, from Vladimir V. Putin in Russia to Kim Jong-un in North Korea, the moral check on dictatorship has disappeared. More and more political operators, not only in former Communist countries in Central Europe, but everywhere, including in some of the oldest democracies, will be emboldened by Mr. Trump’s example. Countries will become more divided between authoritarian populists and the supposed enemies of the people who try to hold them back.

This is the true end of Pax Americana, the best of it anyway. The reckless unpicking of military alliances and the betrayal of allies are already bad enough. But there is something even worse afoot. People all over the world still look to the United States as an example. But now it is the enemies of liberal values who view with glee how American leadership is wrecking the very things that Mr. Roosevelt once fought for.

Ian Buruma, a professor at Bard College, is the author of a book on the rise and fall of the Anglo-American order to be published next year.