But an administration official informed about the discussions privately disputed that account. The 150-dead casualty estimate came not from a general but from a lawyer, according to the official. The estimate was developed by Pentagon lawyers drafting worst-case scenarios that, the official said, did not account for whether the strike was carried out during daytime, when more people might be present at the targets, or in the dark hours before sunrise, as the military planned.

That estimate was passed to the White House counsel, Pat A. Cipollone, without being cleared with Mr. Shanahan or General Dunford. It was then conveyed to the president by the White House lawyers, at which point Mr. Trump changed his mind and called off the strike.

Pentagon lawyers are typically involved in casualty and collateral damage estimates, charged with considering the worst possible outcome. Such numbers are fluid and almost always a rough guess, as it is almost impossible to know who or what will be at the site of an attack when it occurs.

But the lawyers’ involvement was seen by some of Mr. Trump’s aides as an attempt to circumvent Mr. Bolton to influence the president.