James Mattis
Mattis insisted that the U.S. would remain in Syria following the defeat of ISIS to ensure they did not regroup. However, Trump announced the U.S. withdrawal from Syria on December 19. The next day, Mattis submitted his resignation after failing to persuade Trump to reconsider. In his resignation letter, Mattis criticized Trump for not respecting allied nations while criticizing China and Russia's authoritarian rule. In response to the letter, Trump ordered Mattis' firing on January 1, nearly two months earlier than Mattis planned leaving office.