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The calls for revenge came as millions poured into the streets of Tehran to mourn the general, according to state-run news outlets. Iran’s supreme leader, Ayatollah Ali Khamenei, has told advisers that any reprisals must be direct, proportional and openly carried out by Iran — a startling departure for a leadership that typically carries out attacks via regional proxy forces.

How, or even if, Iran may retaliate against the United States is still a matter of speculation. But the crisis had already strained relations between the United States and its European allies — Britain, France and Germany — that were among the nations to strike a now-imperiled nuclear accord with Iran in 2015.

Prime Minister Boris Johnson is walking an especially fine line. Britain will be ever more dependent on its economic relationship with the U.S. after it leaves the European Union later this month. Yet it could also be exposed if tensions arise between the U.S. and the major European powers over Iran.

Related: Amid a rapid U.S. troop mobilization and widespread confusion about the Trump administration’s Iran strategy, Defense Secretary Mark Esper said that a letter about U.S. forces preparing to leave Iraq was a draft that had been released by mistake.

Trump’s threat: Mr. Esper also contradicted President Trump’s earlier threat to strike cultural sites in Iran, acknowledging that doing so would be a war crime. (Our art critics said that would not only be illegal, but also barbaric.)