Under which branch of the military does the Marine Corps fall?

Established as the Continental Marines on November 10, 1775, the Congress commissioned Robert Mullan to raise the first two battalions of Marines, under the leadership of Samuel Nicholas, the first appointed Commandant of the Continental Marines. Their initial purpose was to serve aboard Colonial ships during the Revolutionary War. The Marines quickly became known for the amphibious assault capabilities. Although serving aboard the war ships, they were trained and adept at going ashore and engaging in land battles as well. On March 27, 1794 Congress officially created the United States Navy and Marine Corps and four years later President John Adams established the "Act for establishing and organizing a Marine Corps" which gave the Corps their modern structure.

Since their establishment they have fought in every war and armed conflict of the United States, defending their country on land, sea and air including wars fought on US soil such as the various battles against the First Nations and during the Civil War.

Their motto, "Sempre Fidelis" or Sempre Fi, means "Always Faithful" and represents a way of life for Marines.

This question is dedicated to the 15 Marines and 1 Navy corpsman who died July 11, 2017 in a plane crash in Mississippi.