OVERVIEW: Marine Infantry are amphibious assault troops trained for expeditionary warfare, rapid deployment, and combined-arms operations. They are the core combat element of Marine Corps forces, capable of operating across land, sea, and littoral environments.

DETAILS: Marine Infantry originated in the 18th century as shipboard security detachments and evolved into expeditionary forces specializing in amphibious assaults. Units are organized into infantry battalions and regiments, with squads and fire teams trained in infantry tactics, coastal reconnaissance, and combined-arms integration. Personnel undergo rigorous training in marksmanship, small-unit leadership, amphibious operations, and sustainment under combat conditions. Their doctrine emphasizes mobility, adaptability, and the capability to seize and hold forward positions during initial-entry operations.

FUNCTION: Marine Infantry conduct forcible entry operations by assaulting beachheads, establishing lodgments, and conducting sustained offensive actions inland. They work in coordination with naval gunfire, aviation, armor, and logistics to project power from the sea and secure objectives in contested environments.

NOTE:
IOC: 1775 (establishment of U.S. Marine Corps); analogous units earlier in other nations
Operators: United States, United Kingdom (Royal Marines), France (Troupes de marine), Russia (Naval Infantry), China (Marine Corps), others
Structure: Infantry battalions/regiments within Marine Divisions or Nation-specific Marine Corps
Platforms: Amphibious ships, landing craft, MV-22 Osprey, helicopters, assault and armored vehicles
Conflict used in: American Revolutionary War, World War II Pacific, Korean War, Vietnam War, Gulf Wars, Iraq, Afghanistan, various peacekeeping and maritime security operations

SOURCE: Britannica: https://www.britannica.com/topic/Marine-Infantry ; U.S. DoD Marine Corps Doctrine publications; Wikipedia: https://en.wikipedia.org/wiki/Marine_infantry ; Royal Navy Royal Marines Official website