Secretary of Defense Mattis' resignation letter is absolutely worth reading:
"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.
Because you have the right to 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."

With a potential shutdown looming and Defense Secretary Mattis' departure, this story got buried yesterday: Acting Attorney General Whitaker was told by ethics officials that he should recuse himself from the Mueller investigation but he hasn't.