Back during the 2008 election, when Barack Obama was talking about what he would do with respect to troop levels in Iraq and various other military matters, my rather cynical thought was, “Yeah, if the military lets you do that.”
Sure enough, the military allowed him to do almost none of what he promised to do. He made much of “keeping my promise to bring the troops home from Iraq,” which did happen in 2011, but in actuality he tried for three years to prevent that with the Iraqi government prevailing in the end. American troops have since returned to Iraq.
Congress and two presidents have tried to cut military spending repeatedly. None have succeeded. Congress passed the thing about “sequester,” which automatically cut spending across the board, and somehow military spending still increased.
Obama announced that we would become militarily active in Syria but promised repeatedly that there would be “no boots on the ground” there. Next thing we know there are several thousand troops on the ground in Syria, and at least some of them are in combat.
Trump says that he has ordered the withdrawal of all military from Syria, and we find out soon afterward that the military is actually adding troops there. The next thing we know Trump is saying we will leave several hundred troops in Syria because "everybody agrees it is a great idea."
Two Commanders in Chief have not been able to control the military. Congress cannot control the military. Who’s in charge? The question answers itself.