This article includes the full text of Mattis's letter. It's a really worthwhile read. I'm glad he's speaking out. His default apolitical nature is part of the reason why he commands respect from so many Americans regardless of party affiliation.

Spoken like a true leader - inspiring, unifying and uplifting. These very wise words are what I am deeply craving - substance, maturity, depth, emotional intelligence, wisdom. Twitter has painfully demonstrated how lacking our president is in all the aforementioned attributes. What more proof do we need?

Yep, this is a good one