There is a difference between a politician with a military record and someone who has never served in public office and is then
appointed to office. I'm not saying Mattis will turn the USA into a dictatorship in the blink of an eye. I'm saying that appointing someone
based on his military record into the position of what is supposted to be our civilian oversight of the military is a step in the wrong direction. Mattis is a great battlefield commander, but that's a skill entirely irrelevant for secretary of defense.
In a functioning liberal democracy the military really isn't that powerful-how many men would obey orders to oppose a democratic government in the Western world? How could you practically get unanimous agreement from all the senior figures in the chain of command to issue those orders?
More then you think. Following orders without getting your personal feelings in the way is sorta what you're trained for. Militairy coups don't work with generals going into a big gym of everyone involved and blatantly stating that they're gonna overthrow the government. Often, most if not all of the enlisted men have no idea they're in a coup. You're right, of course - In a
working liberal democracy, the militairy isn't that powerful because it's very carefully monitored by the civilian state.
Even the Polish Home Army, the biggest single resistance force in World War 2, was controlled by a civilian government.