President Trump undermined Rex Tillerson before firing him on Tuesday. He has railed against Attorney General Jeff Sessions on Twitter; chided H.R. McMaster, the national-security adviser, also on Twitter; and pushed back against claims by John Kelly, the White House chief of staff, that the border wall would never be built. But there is one high-profile Cabinet official who has never publicly been on the president’s wrong side: Defense Secretary James Mattis.

On the contrary, Trump has gone out of his way to praise Mattis, singling him out during his State of the Union speech in January for “doing a great job.” Trump has acknowledged that he likes generals—so much so that he named three of them to his inner circle after becoming president: Mattis, McMaster, and Kelly, who was initially the Homeland Security secretary before moving to his White House job. But the military rank is not the only reason why Trump’s relationship with Mattis hasn’t deteriorated as it has with the other generals—and indeed much of the rest of his Cabinet.

As The Washington Post reported last month, “Mattis has been one of the least visible and most consequential members of Trump’s foreign policy team. In Situation Room meetings, he has established himself as a commanding voice, reining in discussions before they devolve into chaos. State Department ambassadors say they have spent more face-to-face time with him than they have their own boss, Secretary of State Rex Tillerson.”