One for the future proves past category. Mattis (and others) were running interference on POTUS's foreign policy agenda. In one instance Mattis ignored Trump's order to move military spouses and children out of harm's way in South Korea.
"In 2017, following a series of North Korean ballistic missile tests, Trump ordered the Pentagon to begin removing the spouses and children of military personnel from South Korea, where the U.S. military has a base. An administration official told the magazine that "Mattis just ignored" the order.
In another instance in the fall of 2017, as White House officials were planning a private meeting at Camp David to develop military options for a possible conflict with North Korea, Mattis allegedly stopped the gathering from happening. He ignored a request from then-national security adviser H.R. McMaster to send officers and planners, according to a former senior administration official."
https://thehill.com/policy/defense/441240-mattis-ignored-orders-from-trump-white-house-on-north-korea-iran-report