That is when @realDonaldTrump told him and entire national security team in the Situation Room that U.S. troops must come home from the civil war in Syria.
Mattis wanted to continue to lie to the public about the mission in Syria.
They used ISIS to justify the real goal, which was regime change.
Mattis only resigned when he lost, and Trump also put Afghanistan on the list.
Mad Dog Mattis was a key player, one of stature, opposed to him keeping that promise, and only left when he could no longer delay.