My thoughts on Mattis:
He was fired by Hussein, so I figured he would be /our guy/.
He decides to resign shortly after Kelly, and after POTUS decides to get out of Syria.
If we are bringing our troops home, perhaps from many places, his command is also lessened. He knows warfare, not diplomacy, as an Anon pointed out.
Curious how POTUS makes a move to leave Syria, and all of a sudden Israel ramps things up. I believe this was a known unknown.
War is a business. How many wars were started to jump start the economy? How many companies manufacture even small items used by our military? More than you can imagine. And those contracts are lucrative. War keeps many businesses and people busy.
Mattis’ job is based, in part, [large part?] on unrest. When a huge part of your job ceases to exist, you look at your job much differently. Are you willing to sit back and wait for things to start to rock and roll again? It's boring - I speak from experience. I don't think it is in his personality to side-line it, even for a short while. POTUS is not warmongering. Maybe this administration’s position is for the long haul, and Mattis is not looking for a sideline job. He doesn't strike me as a benchwarmer.
Are the reasons deeper than not being on the same page as POTUS?
Is this a door Mattis was waiting to be opened so he could use it as an excuse to leave?
Did POTUS install people from Hussein's regime to learn the back story? To get the dirt? To learn the history, see where the corruption was/is, then craft a better position?