Aides said Mr. Trump was angry about a rocket attack last week by forces linked to Tehran that killed an American civilian contractor and stewed as he watched television images of pro-Iranian demonstrators storming the American Embassy in Baghdad in the days that followed, neither of which would normally result in such a seemingly disproportionate retaliation.

But senior officials said the decision to target General Suleimani grew out of a new stream of Iran threats to American embassies, consulates and military personnel in Syria, Iraq and Lebanon. General Suleimani had just left Damascus, the Syrian capital, where he was planning an “imminent” attack that could claim hundreds of lives, officials said…

Still, officials offered scant details and only general explanations for why these reported threats were any different from the rocket attacks, roadside bombings and other assaults carried out by General Suleimani’s Quds Force of the Islamic Revolutionary Guards Corps over the years. “Size, scale and scope,” General Milley said without elaboration.