The United States’ security ties with Europe are fraying. Even before the war in Iran returned the Middle East to the front of policymakers’ minds, the United States was already shifting its focus away from Europe and toward deterring China in the Indo-Pacific. The move is occurring at a perilous moment, with Russia pressuring the continent from its eastern flank as it fights to subjugate Ukraine. As a result, Europeans feel increasingly compelled to take care of their own defense. Donald Trump’s return to the presidency has accelerated this process. He has criticized European countries for insufficient contributions to collective defense and threatened to reduce the number of U.S. forces available to NATO during crises. But the truth is that the underlying shift in transatlantic relations is structural, not personal. Even if future administrations are less hostile to NATO, the United States is unlikely to reverse its broader shift away from Europe.…