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