To President Trump, the question of culpability in the explosions that crippled two oil tankers in the Gulf of Oman is no question at all. “It’s probably got essentially Iran written all over it,” he declared on Friday.

The question is whether the writing is clear to everyone else. For any president, accusing another country of an act of war presents an enormous challenge to overcome skepticism at home and abroad. But for a president known for falsehoods and crisis-churning bombast, the test of credibility appears far more daunting…

“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.