“You have Russia that’s now there. Russia’s on the side of Assad, and Russia wants to get rid of ISIS as much as we do, if not more, because they don’t want them coming into Russia,” Mr. Trump said in a September 2015 interview with CNN. “Let Syria and ISIS fight. Why do we care?”

In a May 2016 interview on MSNBC, Mr. Trump said the United States had “bigger problems than Assad.” He added, “I would have stayed out of Syria and wouldn’t have fought so much for Assad, against Assad.”

So emphatic was Mr. Trump’s stance on Syria that he disavowed the stance of his own running mate. After the October 2016 vice-presidential debate, when Mike Pence, then governor of Indiana, backed strikes against Mr. Assad, Mr. Trump stated, “I disagree.”

In the general election, Mr. Trump repeatedly criticized Hillary Clinton and Mr. Obama for pushing for “immediate regime change in Syria.” Yet as even Mr. Trump noted in his statement on Tuesday, Mr. Obama did little to remove Mr. Assad.

Mr. Trump also disparaged Mrs. Clinton’s campaign stances on Syria — she had denounced Russia’s intervention in the war and called for a no-fly zone — and Mr. Trump said numerous times that her policies would “lead to World War III.”

After his election, Mr. Trump questioned the incentive for ‘attacking.’

“I think going in was a terrible, terrible mistake. Syria, we have to solve that problem because we are going to just keep fighting, fighting forever. I have a different view on Syria than everybody else,” he said during an interview with The New York Times.

Referring to Senator Lindsey Graham’s call to support rebels in Syria fighting Mr. Assad, Mr. Trump said: “Give me a break. I had to listen to Lindsey Graham talk about, you know, attacking Syria and attacking, you know, and it’s like you’re now attacking Russia, you’re attacking Iran, you’re attacking. And what are we getting? We’re getting — and what are we getting?”