Good points @Right_in_Virginia Mattis may have been vain enough to think he could change Trump's mind on those vital issues, but he over estimated his power and underestimated Trump's determination.
@Emjay ALL Mattis has done his whole life is be a cog in a big machine dedicated to fighting wars. First as a paper-pusher in the USMC,and after retirement as a senior paper-pusher in the upper levels of a government stuck on stupid because that's where the money is.
I seriously doubt he even thought for an instant that Trump was serious about any of those things. Were ANY of the previous presidents in your lifetime?
Like anything looks like a nail to a 8 year old boy with a hammer,everything looks like a political and military campaign to a career paper-pusher in the upper levels of the military,and then the civilian government.
His whole career success was based on him telling everyone around them what they wanted to hear,so he would just assume everyone else was lying,too.
"Stuck on Stoopid" is a plainer way to say it.