So, Mattis was fired by Obama. Where was the angst back then? Hypocrites all.

The Weekly Standard:

President Barack Obama fired General James Mattis, the head of Central Command, without even calling the general to let him know he was being replaced.
“I am told that General Mattis was travelling and in a meeting when an aide passed him a note telling him that the Pentagon had announced his replacement as head of Central Command. It was news to him — he hadn’t received a phone call or a heads-up from anyone at the Pentagon or the White House,” Thomas E. Ricks reports.

In another post, Ricks says Mattis was fired because:
Pentagon insiders say
that he rubbed civilian officials the wrong way — not because he went all “mad dog,” which is his public image, and the view at the White House, but rather because he pushed the civilians so hard on considering the second- and third-order consequences of military action against Iran. Some of those questions apparently were uncomfortable. Like, what do you do with Iran once the nuclear issue is resolved and it remains a foe? What do you do if Iran then develops conventional capabilities that could make it hazardous for U.S. Navy ships to operate in the Persian Gulf? He kept saying, “And then what?”
There is also a belief that Mattis and Obama differed on Iran.