The United States (US) Navy and the US Marine Corps have historically enjoyed a very close relationship because the Marine Corps was originally, and continues to be, a part of the Navy. The two, however, operate autonomously, with their own chains of command, uniforms, history, objectives, and missions. Though both the US Navy and US Marine Corps are both seaborne, Marines are often thought of as the foot soldiers of the Navy.
During the Revolutionary War, there was some debate as to whether equipping and supplying a navy was even sensible, given the formidable naval power of the English. Ultimately, the need for some sort of naval force became obvious, leading to the founding of the Continental Navy, which was later disbanded. When pirates started encroaching on the shipping of the nascent nation, the decision was made to establish a formal navy, and the long history of the service began.
Today, the US Navy focuses on keeping a fleet of ships, aircraft, and personnel trained and ready for combat operations, protection of free passage through American shipping lanes, and humanitarian missions. It is headed by the Department of the Navy, a division within the Department of Defense (DOD), and it has installations and active duty personnel all over the world.
The Marine Corps originally started as an infantry branch within the Navy, designed to protect ships from mutinies and to establish beach heads during invasions. Ultimately, the Marine Corps developed into its own service. Although it is technically classified within the Department of the Navy, the head of the US Marine Corps reports to the civilian Secretary of the Navy, not to Navy officials, and the missions of the two forces are different.
Members of the Corps specialize in amphibious warfare, priding themselves on being cross-trained to serve in a variety of positions so that they are versatile and extremely powerful. The US Marine Corps has a strong warrior ethos, and it has historically developed innovative and unique approaches to warfare. Marines are also responsible for the security of the US president in many locations, and they supply guards to US embassies, military bases, and sensitive installations all over the world.
The US Navy and US Marine Corps often work together. It is common for Marines to travel on Naval ships, deploying once they reach their end destination, and members may train with representatives from either force, especially in the case of officers. The Navy supplies corpsmen and chaplains to the US Marine Corps, while the US Marine Corps offers skills and expert support to a variety of Navy operations.
Historically, there has also been some rivalry between the Army and the Marine Corps. The Army feels that the Marine Corps often performs duties that should be performed by the Army, since the Army is supposed to be the premier ground-based military service. Marines argue that their tactical skills in force projection on both land and sea distinguish them from the Army, and Marines have famously paved the way for several Army invasions.