The operation — attacks on a handful of Iranian targets, like radar and missile batteries — was in its early stages when it was called off. Planes were in the air and ships were in position, but no missiles had been fired when word came to stand down.

What we don’t know: It was not clear whether Mr. Trump simply changed his mind or whether the administration altered course because of logistics or strategy.

Context: The retaliation plan was intended as a response to the downing of a $130 million surveillance drone, which was struck Thursday morning by an Iranian surface-to-air missile. Though Iran acknowledged it shot down the drone, the two countries disagree on whether it was in Iranian airspace. See our map.

Big picture: The destruction of the drone underscored the already tense relations between the two countries. Mr. Trump blamed Iran for explosions last week that damaged oil tankers.

Regionally related: The U.S. Senate voted to block a multibillion-dollar arms sale to Saudi Arabia and the United Arab Emirates. The move was a bipartisan rebuke of the president’s unflagging support of Crown Prince Mohammed bin Salman after the killing of Jamal Khashoggi. Lawmakers are unlikely to be able to override President Trump’s expected veto.