In 2020, as racial justice protests swept through the country over the murder of George Floyd, President Trump was itching to deploy the military to crush the unrest. He was talked out of it by his top national security advisers, who feared that such a decision would be viewed as moving toward martial law.

Five years later, as protests against his immigration policies began to swell in Los Angeles, Mr. Trump said he had learned his lesson.

“I’ll never do that again,” Mr. Trump said on Thursday, about waiting to send in the National Guard in 2020. “If I see problems brewing,” he added, “I’m not going to wait two weeks.”

With the Los Angeles protests, Mr. Trump has seized the chance to make up for his first-term regret.

His decision to send in federal troops right away, taking the extraordinary step of deploying active-duty military to deal with domestic unrest, fits into the larger pattern of Mr. Trump operating without any significant pushback from the people around him in his second term.

“He saw the military as his reactionary arm,” said Olivia Troye, a former homeland security official and aide to former Vice President Mike Pence. Ms. Troye said she witnessed multiple national security officials explain to Mr. Trump in 2020 that the military takes an oath to the Constitution — not Mr. Trump — and that it should not be turned against American citizens, even protesters.