The appearance of erratic decision-making “adds to the confusion of his allies and adversaries,” said Nigel Sheinwald, a former British ambassador to Washington who once conducted hostage negotiations with the Iranian government.

But it is also part of “a continuing picture of American uncertainty about the use of power,” Mr. Sheinwald said. “It is the leitmotif that runs throughout U.S. history.”…

“Trump’s rather unconventional decision-making style may have saved the day,” said Crispin Blunt, a senior Conservative lawmaker in Britain who is a former head of Parliament’s foreign affairs committee.

“I suspect other presidents, having approved the mission, would not have aborted for fear of having to give a public explanation that plainly suggests inadequate consideration in the first place,” added Mr. Blunt, who — like the British government itself — opposes Mr. Trump’s policy on Iran. “The strength of the war lobby with a line to him should not be underestimated, and I’m relieved he faced them down, if dangerously late in the day.”