The New York Times | Peter Baker : To President Trump, the question of culpability in the explosions that crippled two oil tankersin 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.


For two and a half years in office, Mr. Trump has spun out so many misleading or untrue statements about himself, his enemies, his policies, his politics, his family, his personal story, his finances and his interactions with staff that even his own former communications director once said “he’s a liar” and many Americans long ago concluded that he cannot be trusted.


Fact-checking Mr. Trump is a full-time occupation in Washington, and in no other circumstance is faith in a president’s word as vital as in matters of war and peace. The public grew cynical about presidents and intelligence after George W. Bush’s invasion of Iraq based on false accusations of weapons of mass destruction, and the doubt spilled over to Barack Obama when he accused Syria of gassing its own people. As Mr. Trump confronts Iran, he carries the burden of their history and his own.


“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.