Yes, Good old Mac was in the Pacific. That was his theater of Ops to be responsible for. The Marine Corps is a Dept. of the Navy, so we pretty much stick to amphibious warfare... soldiers of the sea. There were some Army troops in the area, just as there were a few U. S Marines fighting Hitler. However for the most part, Europe belonged to the Army, and the Pacific belonged to the Marines.
Iceland, due to its strategic location needed secured back in the Spring of 41 before the Japs hit us. It was a British Garrison, however Churchill didn't have the force needed to protect it. The U. S. Army said they could do it, but it would take some time to get there and get set up. The President called on the Marines needing a fast response. They took off in late June and arrived shortly after the 4th of July. To this day we still provide the security for Iceland. It was an amphibious ordeal, that's what we do, so they used us.
The Pacific islands were "raided" by Marines in amphibious assaults. Once the enemy was closed with and destroyed, alot of the Army troops went in to do construction work, build landing strips, bury the dead and the like. The islands were not just taken to kill Japs, we needed them for air bases and "safe havens". The Marines in Europe were there primarily to train the Army guys on amphibious landings and beach assaults.
During the invasion of Okinawa, there was more of a dual Army/Marine fighting force. The Marines were prepared to fight bears with flashlights, but the Japs had decided to lure everyone in, and decided not to oppose the landing. The Japs saw this as the last hurrah, and unleashed more kama-kazis than ever, in waves up to 350 planes strong at times. The U. S. Navy was trying like hell to provide support for the Marines and Army troops on shore who were fighting very fierce Japs, all the while trying to fight the Japs as well. The U. S. Navy lost 34 ships, 368 damaged, 9000 Sailors dead. The greatest losses ever sustained by the Navy in a single battle. Just a bit of history.