America officially entered WW2 after the bombing of Pearl Harbour on December 7th 1941. U.S. forces had been involved in various incidents with both the Japanese and Chinese since 1937 also U.S. Marine personnel had been stationed in Britain in an unofficial capacity since the outbreak of war in 1939.
The duties they carried out in Britain during this early period 1939-1941 were primarily guard to the U.S. Embassy in London. Their other role was one of liaison with the Royal Air Force (RAF) – to gain intelligence on aerial warfare and the Royal Marine Commandoes.
In Europe during the period of war they garrisoned Iceland against possible German invasion, thereby relieving British troops to other duties. They were part of the Office of Strategic Studies (OSS) and were dropped behind enemy lines to liaise with resistance groups. They fought along the coast of North Africa and after D Day (although performing many and varied tasks) they had responsibility for taking charge of enemy prisoners and recovery and protection of stolen artefacts and art treasures. Operation Dragoon in August 1944 saw them landing in Southern France where they participated in the capture of Marseille.
They are more famously know for the Pacific Campaign defending posts on places like Wake Island and the Philippines before being ousted by the Japanese (late 1941 and early 1942). In late 1942, alongside the other services, they landed at Guadalcanal; starting the long campaign that would take them across the Pacific to the gates of Japan. The dropping of two atomic bombs brought about the end of the war after which the Marines became part of the occupation force.
The above is just a very brief synopsis of the USMC involvement in WW2. There are many sites that will supply further detail or you can contact us for more information.