Secretary of State Mike Pompeo and Vice President Mike Pence were two of the most hawkish voices arguing for a response to Iranian aggression, according to administration officials. Mr. Pence’s office helped run herd on meetings and conference calls held by officials in the run-up to the strike.

Defense Secretary Mark T. Esper and General Milley declined to comment for this article, but General Milley’s spokeswoman, Col. DeDe Halfhill, said, without elaborating, that “some of the characterizations being asserted by other sources are false” and that she would not discuss conversations between General Milley and the president.

The fallout from Mr. Trump’s targeted killing is now underway. On Saturday in Iraq, the American military was on alert as tens of thousands of pro-Iranian fighters marched through the streets of Baghdad and calls accelerated to eject the United States from the country. United States Central Command, which oversees American military operations in the Middle East, said there were two rocket attacks near Iraqi bases that host American troops, but no one was injured.

In Iran, the ayatollah vowed “forceful revenge” as the country mourned the death of General Suleimani.

In Palm Beach, Fla., Mr. Trump lashed back, promising to strike 52 sites across Iran — representing the number of American hostages taken by Iran in 1979 — if Iran attacked Americans or American interests. On Saturday night, Mr. Trump warned on Twitter that some sites were “at a very high level & important to Iran & the Iranian culture, and those targets, and Iran itself, WILL BE HIT VERY FAST AND VERY HARD.”

The president issued those warnings after American spy agencies on Saturday detected that Iranian ballistic missile units across the country had gone to a heightened state of readiness, a United States official said on Saturday night.

Other officials said it was unclear whether Iran was dispersing its ballistic missile units — the heart of the Iranian military — to avoid American attack, or was mobilizing the units for a major strike against American targets or allies in the region in retaliation for General Suleimani’s death.