From Jonathan Bernstein:
Had Mattis merely singled out Syria or Afghanistan policy, he might’ve divided Trump’s opponents, some of whom are very much in favor of bringing troops home. Instead, he aligned himself with the broader foreign-policy consensus of those who support NATO and other international arrangements, leaving Trump on the fringes. In historical terms, Mattis is with Franklin Roosevelt, Harry Truman and Dwight Eisenhower; Trump is with (at best) Henry Wallace or the Birchers.
It’s not clear that Trump realizes the risks he now faces. He had already been losing influence with congressional Republicans since the midterms, as expected. The more dangerous the world looks to members of Congress from both parties, the better Vice President Mike Pence looks. And Trump has furnished Congress with plenty of excuses to remove him if they otherwise want to; within a few months, it may be hard to avoid the question of impeachment and removal even for those who don’t particularly want to do it.