This means that sometimes—frequently, even—American presidents will be making decisions that either help foreign leaders stay in power or create political problems for them at home. Those leaders will reciprocate: Mr. Xi can either buy or not buy goods from U.S. farm states in the same way that Mr. Trump can either help or hinder Huawei.

This is about more than concerns over Mr. Trump’s conflicts of interest. As the Halkbank problem shows, the realities of foreign policy may force American presidents to assert more political control over sensitive matters like Justice Department investigations of foreign-owned firms. It is inevitable that some presidents will be tempted to abuse this authority; it is undeniable that all presidents will need it.

The so-called imperial presidency, developed in the 1960s as the Cold War necessities of global engagement, concentrated decision-making power in the Oval Office. Even as Mr. Trump’s erratic approach to the presidency illustrates the dangers of executive power, the new shape of world politics is conspiring to make the U.S. presidency more powerful still.