I love the military of the United States.  There are currently five branches: the Army, Marine Corps, Navy, Air Force, and Coast Guard.  President Trump just announce a new branch called the Space Force yesterday.
I heard about that! Do you think the Space Force will fight aliens? Or attack countries in space?
LOL.  My guess would be a space fight against other countries.  The President is the commander-in-chief of all our armed forces.  And the Department of Defense really impliments his plans.  
It is really interesting to me how the President automatically becomes commander-in-chief, even without any military experience.
That is part of the role of the President.  Thankfully, though commander-in-chief, most Presidents rely on their military leaders to assist in planning and give direction.  Ultimately, however, all decisions of the military come down to the commander-in-chief  
What else can a commander-in-chief do? 
Well, for the United States President, it refers to his role as the nation's executive leadership.  He is the head of the State (our Union), and the head of the Government's executive branch, which is the enforcement arm of the three branches.