A marine is a member of a force that specializes in expeditionary operations, such as amphibious assault and occupation. Marines traditionally have strong links with the country's navy. In operation the commander in the field is usually the commander of the fleet; services are provided by the navy (medical, legal, jails, supply); the anchor is often present as a symbol (USMC, French Troupes de Marine, Dutch Korps Mariniers), and the name marine means "navy" in French, Dutch, and German. However, the marine force is the department of the navy, and can operate under the army or as an independent command.

Historically tasks undertaken by marines have included providing protection from war while at sea, reflecting the pressed nature of the ships' company and the risk of mutiny. Other tasks would include boarding of vessels during combat or capture of prize ships and providing manpower for raiding ashore in support of the naval objectives. Marine elements would also contribute to the campaign ashore, in support of the military objective.

With the industrialization of warfare in the 20th century the scale of landing operations increased; thus brought with it an increased likelihood of opposition and a need for co-ordination of various military elements. Marine forces evolved to specialize in the skills and capabilities required for amphibious warfare.

In the earliest days of naval warfare, there was little distinction between sailors and soldiers on a warship, as the oarsmen of Ancient Greek and Roman ships had to be capable of fighting the rowers of opposing ships hand-to-hand; though hoplites began appearing on Greek ships specifically for the boarding of enemy ships. The Roman Navy's two legions, I Adiutrix and II Adiutrix, were among the first distinct naval infantry units.