The United States Marine Corps, separate military service within the U.S. Department of the Navy, charged with the provision of marine troops for seizure and defense of advanced bases and with conducting operations on land and in the air incident to naval campaigns. It is also responsible for providing detachments for service aboard certain types of naval vessels, as well as security forces for naval shore installations and U.S. diplomatic missions in foreign countries.
The Marine Corps was founded on November 10, 1775, when the Continental Congress ordered that two battalions of Marines be raised for service as landing forces with the fleet. Marines have participated in all wars of the United States, being in most instances first, or among the first, to fight. In addition, Marines have executed more than 300 landings on foreign shores and served in every major U.S. naval action since 1775.
The U.S. Marine Corps is structured according to the National Security Act of 1947 and its amendments of 1952. The commandant of the corps has coequal status with members of the Joint Chiefs of Staff in all matters pertaining to the corps. The corps is composed of two operating forces, the Fleet Marine Force, Pacific (FMFPAC) and the Fleet Marine Force, Atlantic (FMFLANT); a supporting establishment for recruitment, training, supply logistics, and maintenance of bases, installations, and schools; and the Marine Corps Reserve. As of 2017, the USMC has around 186,000 active duty members and some 38,500 personnel in reserve.
According to en.wikipedia and britannica.com