PALM BEACH, Fla. — The decision came Thursday afternoon on Air Force One on the way to Florida. President Trump assembled his National Security Council on his plane, some by secure video link, as the generals made the case that President Bashar al-Assad of Syria had to learn there was a price to pay.

Mr. Trump was already shaken by photos his staff had shown him of children dying after the Syrian government’s chemical weapons attack — far more graphic than those the public had seen — so the president did not need a lot of convincing.

“What happened in Syria is truly one of the egregious crimes, and it shouldn’t have happened,” he told reporters when he poked his head into the press cabin. “And it shouldn’t be allowed to happen.”

Two hours later at his Mar-a-Lago resort in Palm Beach, Fla., Mr. Trump gave the order to unleash 59 Tomahawk cruise missiles at Al Shayrat airfield in western Syria, where the chemical weapons attack originated. His generals had given him the option of delaying a day, but Mr. Trump chose not to wait.

It had been only 63 hours from the chemical attack to the American strike.

Early morning, Tuesday, April 4