Because if this is what Mattis has been thinking to himself he never should have accepted the job.
As a retired veteran, I think a person owes their commander honesty and loyalty. It’s OK to not agree with the boss. One is obligated, however, to bring up those disagreements and state one’s case. Once the final decision is made, it’s up to the subordinate to carry out the commander’s policies to the absolute best of one’s ability.
This is where I have a problem with Mattis. From his resignation, it sounds like he fundamentally disagrees with the CINC on major issues. It also appears he slow walked Trump’s policies for the last two years (although I don’t have proof of that). If one cannot carry out the boss’s wishes to the best of one’s abilities, resign and get out of the way.
An honorable person, I think, wouldn’t even take the job in the first place if they knew, as Mattis should have, that they didn’t agree with what Trump campaigned on. Americans are tired of being Team America World Police, and Trump promised to end all these foreign wars and police actions.
While I've always been a fan of Mattis, and agree with much of what he said, it was petty, unprofessional and insubordinate to publicly air his dissension with the CINC. He is basically saying he can't work for a CINC who doesn't provide effective leadership to our alliances, nor treat our allies with respect, then goes on to lecture the President about the geopolitical ambitions of Russia and China. This is the kind of stuff you say, very bluntly but respectfully, behind a closed door; not in your public resignation letter. This was a bit of a knife in the back.
I suspect this is strategic. This is Mattis publicly distancing himself from Trump, and what he sees as future failures of US international and geopolitical goals. In reality, the "new world order" of the past two decades has so damaged our economy and military, Trump has a near impossible task to prevent a coming decline. Mattis knows Trump, and by extension he himself, will be blamed. Of course, should Trump succeed, Mattis still looks great. So, distance...
Around six years ago, I was in a conversation with a Army vet who used the term ‘the forever wars’...which meant that the US military had turned into a policing organization, and the only way for an officer to advance....was to participate and get noted for Operation-such-and-such. I think a lot of the junior officers who went through the 1990s fell for the concept and Mattis is just one of those left over from that era.
I can’t think of a single situation other than the Korean peninsula where US military operations should be deployed outside of the fifty states. Even in NATO...there’s no threat and we should just shut down and bring the forces back home. If Kim could reach some agreeable peace arrangement...I could even see US forces returning from South Korea.