“The problem is twofold for them,” said John E. McLaughlin, a deputy C.I.A. director during the Iraq war. “One is people will always rightly question intelligence because it’s not an exact science. But the most important problem for them is their own credibility and contradictions.”

The task is all the more formidable for Mr. Trump, who himself has assailed the reliability of America’s intelligence agencies and even the intelligence chiefs he appointed, suggesting they could not be believed when their conclusions have not fit his worldview.

At one point shortly before taking the oath of office, he compared intelligence agencies to Nazi Germany and ever since has cast doubt on their findings about Russia’s interference in the 2016 election. This year, he repudiated his intelligence chiefs for their assessments of issues like Iran, declaring that “they are wrong” and “should go back to school.” And just this week, he rebuked the C.I.A. for using a brother of North Korea’s Kim Jong-un as an informant, saying, “I wouldn’t let that happen under my auspices.”

All of that can raise questions when international tension flares up, like the explosion of the two oil tankers on Thursday, a provocation that fueled anxiety about the world’s most important oil shipping route and the prospect of escalation into military conflict. When Mr. Trump told Fox News on Friday that “Iran did do it,” he was asking his country to accept his word.

“Trump’s credibility is about as solid as a snake oil salesman,” said Jen Psaki, who was the White House communications director and top State Department spokeswoman under Mr. Obama. “That may work for selling his particular brand to his political base, but during serious times, it leaves him without a wealth of good will and trust from the public that what he is saying is true even on an issue as serious as Iran’s complicity in the tanker explosions.”