Mattis quietly keeps distance from Trump's most divisive moves

Mr. Mattis is trying to ensure he keeps his line to the president open so Mr. Trump can quickly be briefed on the issues that Mr. Mattis cares about the most. When the Syrian government launched a chemical weapons attack in April, Mr. Mattis rapidly presented his boss with possible responses; within 63 hours, a hail of missiles were fired on a Syrian airfield, the option Mr. Trump had chosen.

He also heeds entreaties from the president. Mr. Mattis held a May 19 news conference on progress in the campaign against the Islamic State after Mr. Trump asked him to do it. Mr. Mattis used the occasion to assert that the idea of surrounding enemy strongholds to prevent Islamic State fighters from escaping and to “annihilate” the group was originated by the president, who did not serve in the armed forces and has no experience as a military strategist.

But people close to Mr. Mattis said that he distinguished between requests from Mr. Trump himself and those from administration aides, who have called the Pentagon weekly suggesting that he goes on “Fox & Friends,” the Fox News morning show that is supportive of Mr. Trump, for instance, but to no avail.