Mattis' resignation letter was juvenile & extremely unprofessional and a thinly veiled attempt to poke POTUS in the eye (50 copies made before-hand to ensure max publicity)
The tone and tenor of his letter is something seen written by Jr company grade officers (lieutenant, young captain), but any semi-mature military leader is ingrained with "your correspondence carries zero weight when written in an emotional tone."
Mattis has 40 years writing military correspondence & has a large staff of people that write speeches and correspondence for him…he KNEW this was wrong & his entire staff KNEW this letter was the wrong thing to do, and would bring discredit upon the entire DoD
POTUS had NO CHOICE but to fire Mattis (for the sake of all those in uniform)