What did the marine soldiers do during World War 2?

Answer 1

For the US, the enemy (Germany & Japan) were divided into two basic theaters; the ETO (European Theater of Operations) and PTO (Pacific Theater of Operations). There were more theaters, MTO (Mediterranean Theater of Operations), CBI (China-Burma-India Theater of Operations), etc. But the "basic" (primary ones) ones were the ETO and PTO.

The ETO was a land war; the PTO was an ocean war. The US Army has jurisdiction on the land. The US Navy has jurisdiction on the seas.

Although the US Army did have units in the PTO (and CBI etc.); since the PTO was a naval war, the primary combatant was the US Navy.

The US Marine Corps are part of the US Navy. The marines are "naval infantry" (or used to be during the 19th & 20th centuries). When the USN needed a base (an island for example), the marines had to secure for them (take it for them). Consequently, the USMC had a lot of islands to fight for in the USN's drive towards Japan in the PTO.

In the ETO, the US Army simply drove towards Berlin, there was no ocean between Berlin and the US Army. There were also very few US Marines in the ETO, as there was no need for US Naval bases to be taken.