Syria’s use of chemical weapons violated international norms, while Iran’s actions were rooted in Mr. Trump’s decision in May 2018 to pull out of the Iran nuclear deal and impose sanctions on Iran, driving down its oil exports and throttling its economy.
It is, to a great extent, a crisis Mr. Trump manufactured.
How well he weathers the crisis depends on how Iran responds. In Mr. Obama’s case, the dismay over his failure to enforce his red line was mitigated when Russia subsequently proposed another way to rid Syria of its chemical weapons, by handing them over to an outside authority. This allowed Mr. Obama to withdraw his request for congressional backing of a military strike.
“It’s doubtful that Trump will draw some benefit in terms of Iranian behavior from backing down,” said Martin S. Indyk, a former American ambassador to Israel. “But if he does — for example, if it produces negotiations — then this difference turns into a parallel.”
Defenders of Mr. Trump dismissed the comparison to Syria, saying that Mr. Obama sought congressional authorization, knowing full well that it would never be granted. His calculation, they said, was driven by a risk aversion that grew out of his bitter experience with the American-led intervention in Libya. Mr. Trump, they noted, ordered two missile strikes on Syria after it used chemical weapons.
“Trump is not risk averse,” said James Jay Carafano, a foreign policy expert at the Heritage Foundation who advised Mr. Trump during the transition. “He is perfectly willing to use force. He is not afraid about getting into an escalatory conflict, but his use of force is calculated, and he predetermines he is not going to get on an escalatory path.”
Still, one thing Mr. Trump and Mr. Obama share is a conviction that the United States has blundered into too many foreign conflicts. For all his bluster — and his methodical dismantling of his predecessor’s legacy — Mr. Trump has so far avoided being drawn into a major military entanglement.
Jake Sullivan, a former senior adviser to Hillary Clinton and Vice President Joseph R. Biden Jr., said that Mr. Trump and Mr. Obama both seemed driven by the “common concern about what more they would be buying in the way of military engagement, beyond a one-off retaliatory strike.”