Trump is said to divide the members of his Cabinet into first-tier “killers” and second-tier “winners.” Mattis is indisputably a killer, but he’s also something rarer: a sometime loser — of policy arguments, that is — who manages to disagree with the president without squandering his clout or getting under Trump’s skin. He opposed Trump’s decision to withdraw from the Paris climate change accord, decertify the Iran deal, slap tariffs on steel and aluminum, and move the U.S. Embassy in Israel from Tel Aviv to Jerusalem. He opposes the president’s proposed ban on transgender service members and has reportedly ignored requests from the White House to see plans for a military strike against North Korea.

Yet Mattis has been able to present the president with views he doesn’t like without bearing the brunt of his frustration. The departure of H.R. McMaster, his national security adviser, was announced Thursday amid rumors that the president is poised to fire beleaguered Cabinet secretaries like David Shulkin of Veterans Affairs and Ben Carson of Housing and Urban Development, and is agonizing over whether to dismiss John Kelly, his chief of staff. Mattis’ name has been conspicuously absent. One senior administration official called him “bulletproof.”…

Even his detractors on the right are reluctant to criticize Mattis, a retired Marine general who fought in both Gulf wars and Afghanistan, on the record, either because they are looking to join the administration or because, despite their disagreements, they are happy he’s there. Those critics say privately that Mattis is too cautious and conventional in his thinking, and that he doesn’t sufficiently appreciate the political nature of his current job. But “they regard him as better than the likely alternatives,” one such critic told me, and consider him “a restraining hand on an otherwise unpredictable and impulsive president.”