One of the central lessons of international relations is that war is easy, but peace is hard. The 70 years of relative peace and prosperity that most countries have enjoyed since the end of the Second World War did not just happen. They resulted from the conscious efforts of the United States to construct and sustain an open, rule-based, liberal world order — a world order that is now at risk under President Donald Trump’s “America First” doctrine.

To protect America’s global interests required the U.S. to build a military strong enough to impose unacceptable costs on any who might harm them. But a United States powerful enough to dissuade its enemies on a worldwide basis could also use that power to prey on its friends. To allay these fears, the U.S. both embedded its power within rules, norms and institutions that constrained it from acting arbitrarily in ways that harmed its allies’ vital interests, and legitimated its power by binding its use to an overarching, shared moral purpose — the protection of human rights and the promotion of human liberty.

The U.S. was not motivated by altruism. The United Nations Charter was signed in San Francisco, after all, not neutral Geneva. The U.S. saw an open, liberal international order centered upon American power as the best way to protect American security and American interests in an otherwise hostile world. Strong alliances with other liberal regimes were necessary because no single state, not even the U.S. during its nuclear monopoly, is powerful enough to generate a stable global order in a world of fragmented political authority. And because an American-centric liberal world order would also protect and promote the interests of other liberal regimes, it offered a potential escape from the deadly, dog-eat-dog, balance-of-power politics and two world wars that killed up to 100 million people in the first half of the 20th century.

President Trump’s foreign policy of “America First” seeks a different pathway to protect American interests. It (correctly) perceives that other countries, especially American allies, have reaped substantial benefits from an open, liberal world order that imposes real constraints on the U.S., and that American allies do sometimes free ride on the liberal order that American power sustains. But rather than tolerating some free-riding as unavoidable, and the constraints on American power as necessary to building trust, good will and common purpose among allies, Trump sees both as ways in which others shamelessly exploit the United States’ (naïve) good will.

Trump’s “America First” policy accepts the liberal order’s premise that protecting America’s global interests requires an American military strong enough to dissuade any who might harm it, but it rejects any constraint on the exercise of American power in the pursuit of American interests. Trump’s rhetoric and actions, such as the metastasizing trade war with our major trading partners or his threat to abandon NATO, intentionally seek to undermine the core economic and security institutions that structure the liberal world order precisely because these institutions constrain the United States from using its power to prey on others, or, in more Trumpian language, “to cut a better deal.”

Trump likewise rejects the notion that American power should be bound to any overarching moral purpose apart from protecting America’s narrow self-interest. He thus disparages the United States’ traditional democratic allies, while praising unconstrained, authoritarian strong men such as Turkish President Recep Tayyip Erdog˘an, North Korean Supreme Leader Kim Jong Un or Russian President Vladimir Putin. “It is the right of all nations,” Trump proclaimed in his inaugural address, “to put their own interests first. We do not seek to impose our way of life on anyone. … At the bedrock of our politics will be a total allegiance to the United States of America.”

In effect, Trump seeks to return the U.S. to a world of balance-of-power politics that the architects of the liberal order sought to escape. It is a world of unconstrained power ordered by fear rather than trust, in which the powerful do what they will, while the weak suffer what they must. Such a world may well yield short-term gains, as other countries make concessions to protect themselves from an aggressive and opportunistic United States. But the long-term prognosis is grim.