Thursday, Mattis wrote in a resignation letter that he felt he could not carry on in a position where he was serving policies which he himself would not have supported or chartered.

“Because you have the right to have a secretary of defense whose views are better aligned with yours … I believe it is right for me to step down from my position,” he wrote.

Mattis’ letter cited his support for America’s “unique and comprehensive system of alliances and partnerships.” Trump’s differing views on how to approach those international relationships, Mattis wrote, is one reason he feels the President deserves a Secretary of Defense that shares his opinions.

General Mattis is sure to be remembered by his many years in public service, and of course, his nicknames earned through his ruthless and calculating reputation: “warrior monk”, “Mad Dog”, and “Chaos”.

John Dickerson of CBS News, for example, once asked Mattis in an interview what scared him or kept him up at night.

“Nothing,” he responded. “I keep other people up at night.”