For decades, U.S. foreign policy has depended on credibility: the belief that Washington would honor its commitments and that its past behavior signaled its future conduct. The United States, for instance, was able to develop a large network of allies because its partners trusted that, if attacked, Washington would defend them. It could strike free-trade deals with countries around the world and negotiate peace agreements because, generally speaking, it was seen as an honest broker. That is not to say the United States has never surprised, or that it never reneged on a promise. But for most of its modern history, it has been a trustworthy actor.

But unlike any U.S. president before him, Donald Trump has abandoned all efforts to make Washington reliable or consistent. His predecessors had also, at times, made decisions that undermined American credibility. But Trump’s lack of consistency is of an entirely different magnitude—and appears to be part of a deliberate strategy. He proposes deals before backing down. He promises to end wars before expanding them. He berates U.S. allies and embraces adversaries. With Trump, the only pattern is the lack of one.

Trump’s theory of the case is simple. By keeping friends and foes off balance, the president believes he can secure quick wins, such as modest increases in European defense spending. Trump also thinks that unpredictability affords him greater wiggle room in international affairs by ensuring that allies and adversaries are always second-guessing his next course of action. Finally, Trump thinks that he can frighten and thus deter opponents by appearing unhinged—an idea that political scientists call the madman theory. As Trump once boasted, Chinese President Xi Jinping would never risk a blockade of Taiwan while he is president because Xi “knows I’m fucking crazy.”

As some analysts have pointed out, Trump’s approach has delivered a few temporary international victories. But in the long term, Trump’s approach to global politics is not likely to strengthen the country. Other states will work to flatter Washington for a time, in hopes of avoiding U.S. penalties. But eventually, governments will look to protect themselves by aligning with other countries. The United States’ list of adversaries will, accordingly, grow. Its alliances will weaken. Washington, in other words, could find itself ever more isolated—and without any clear path to reestablishing its reputation.

U.S. presidents have consistently argued that, to protect American power, Washington’s commitments need to be credible. Harry Truman, for instance, decided to intervene on the Korean Peninsula in order to check Soviet expansionism. “I remembered how each time that the democracies failed to act, it had encouraged the aggressors to keep going ahead,” he later said, by way of explanation. Lyndon Johnson escalated the war in Vietnam largely out of fear that retreat would signal that Washington wasn’t serious about containing communism. George W. Bush justified the 2007 surge in Iraq on the grounds that withdrawal would undermine U.S. credibility; Barack Obama kept U.S. forces in the country for similar reasons. And when Obama hesitated to enforce his self-proclaimed “redline” against chemical weapons use in Syria, he was pilloried by his critics for emboldening the United States’ enemies. (He later told a journalist that “dropping bombs on someone to prove that you’re willing to drop bombs on someone is just about the worst reason to use force.”) After the botched withdrawal from Afghanistan, Joe Biden was also criticized for undermining Washington’s reputation for reliability, resolve, and competence.

What effect these leaders’ decisions actually had on American credibility is unclear. The causal relationship between a state’s decisions and the way those decisions are perceived is complex and fuzzy. The Dartmouth political scientist Daryl Press, for instance, has argued that states look to present interests and........

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