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 America na 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.