Q. Are the Marines a branch of the Navy or are they their own military department?
The United States Marine Corps is a branch of the U.S. Department of the Navy.
However, as you are probably aware, the Marines are often used on missions that seemingly have nothing to do with naval maneuvers. While the Marines were formed to serve as security on board Navy ships, their use has evolved over the years to include a variety of missions, including “expeditionary” missions, which basically means any sort of short-term mission with a particular purpose, anywhere in the world.