The wars and insurgencies that battered the Middle East over the last decade delivered not only a horrible toll of death and displacement, but also a wasteland of cultural destruction, reducing to rubble the Assyrian gates of Nineveh, the Great Mosque of Aleppo and countless other treasures, ancient and modern.

This past weekend the American commander-in-chief, scrambling to contain the fallout from the killing of Maj. Gen. Qassim Suleimani of Iran, proclaimed via Twitter that “if Iran strikes any Americans,”‌ the United States would retaliate by blasting a list of 52 Iranian sites that he said were “important to Iran & the Iranian culture.” Historians, legal scholars, and Democrats such as Senator Elizabeth Warren noted that targeting sites of cultural importance in this way would constitute a war crime — and even Secretary of State Mike Pompeo went out of his way to clarify on Sunday morning television that “the American people should know that every target that we strike will be a lawful target.”

But Mr. Trump reaffirmed that he saw culture as fair game. “They’re allowed to use roadside bombs and blow up our people,” the president told reporters on Air Force One. “And we’re not allowed to touch their cultural site? It doesn’t work that way.”

Despite the president’s threats, Defense Secretary Mark T. Esper on Monday explicitly ruled out targeting cultural sites in Iran in response to any Iranian attacks.