What is the Marine Corps?

The United States Marine Corps (USMC) is a branch of the U.S. military, responsible for providing power projection from the sea, utilizing the mobility of the U.S. Navy to rapidly deliver combined-arms task forces to crises around the globe. Along with the U.S. Navy, it falls under the United States Department of the Navy.
Originally organized as the Continental Marines in 1775 as naval infantry, the Marine Corps would evolve its mission with changing military doctrine and American foreign policy. Owing to the availability of Marine forces at sea, the Marine Corps has served in every conflict in U.S. history. It attained prominence when its theories and practice of amphibious warfare proved prescient, and ultimately formed a cornerstone of the Pacific campaign of WWII. By the early 20th century, the Marine Corps would become the dominant theorist and practitioner of amphibious warfare. Its ability to rapidly respond to regional crises has made and continues to make it an important tool for American foreign policy.
The Marine Corps, with 180,000 active and 40,000 reserve Marines as of 2005, is smaller than the U.S. Army, U.S. Navy, and U.S. Air Force. Only the U.S. Coast Guard is a smaller military service than the Marines.