By taking the steps you propose, Mattis would promptly eliminate his ability to do any good, and likely fail at preventing the harm he is trying to prevent. An

By taking the steps you propose, Mattis would promptly eliminate his ability to do any good, and likely fail at preventing the harm he is trying to prevent. And yes, if the person occupying the President's chair had actually given a specific direct order, as in "As President, I'm hereby ordering you specifically to implement a plan to attack and kill another head of state, specifically Bashir Al Assad; do it by next week or I'll relieve you of duty and court martial you for insubordination", Mattis would have little choice to either comply, resign, or wait for the MPs to arrest him. However, it is extremely unlikely that the order was that specific, and so both creates the moral dilemma referenced above, and the opportunity to use discretion in how the request is implemented. And even if such a "Big Fucking Deal" happened, in this environment, it's a fart in a hurricane, and if you actually expect this congress to act as you expected, perhaps we should talk about this bridge I have for sale...