There are only three sentences in Defense Secretary Jim Mattis' resignation letter you need to read in order to understand why he's stepping down.
After reading plenty of White House/Pentagon palace intrigue stories, we know the relationship between Mattis and President Donald Trump has waned in recent months. And Mattis' letter makes it abundantly clear of why: He fundamentally disagrees with the president on the path forward.
“My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades of immersion in these issues. We must do everything possible to advance an international order that is most conducive to our security, prosperity and values, and we are strengthened in this effort by the solidarity of our alliances,” Mattis wrote.
“Because you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position.”
Put another way, Mattis is contrasting Trump's views with his own: The president doesn't respect allies nor is he clear-eyed about who are enemies are (looking at you, Russia). Trump also doesn't care much about the international order built after World War II that helps keep the world fairly secure.
Mattis' resignation comes a day after Trump decided to pull U.S. troops from Syria, apparently without much input from DoD. Trump is also mulling whether to pull a substantial number of American troops from Afghanistan within the next several weeks, according to a new report in The Wall Street Journal.