This was/is a huge clue:
https://www.military.com/daily-news/2018/12/20/defense-secretary-james-mattis-retire-end-february-trump-says.html
Mattis Resigns Following Trump's Syria Withdrawal Order
Defense Secretary Jim Mattis resigned Thursday, seemingly in protest of President Donald Trump's orders to withdraw from Syria and an "America First" policy that appears to discount the value of allies and local partnered forces fighting alongside U.S. troops.