The United States Marine Corps 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 global crises. Alongside the U.S. Navy, the Marine Corps operates under the United States Department of the Navy. Originally organized as the Continental Marines, the Marine Corps has evolved its mission with changing military doctrine and American foreign policy. The Marine Corps has served in every American armed conflict including the Revolutionary War. It attained prominence in the twentieth century when its theories and practice of amphibious warfare proved prescient, and ultimately formed the cornerstone of the Pacific campaign of World War II. By the early twentieth century, the Marine Corps had 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 body in the implementation and execution of American foreign policy. The United States Marine Corps, with 180,000 active duty and 40,000 reserve Marines as of 2005, is the smallest of the United States' armed forces in the Department of Defense.