18th Marines was activated on 8 September 1942 and participated in actions at Tarawa, Saipan and Tinian.
It was inactivated on 16 August 1944.

On Tinian there were two small beaches on the North end where an assault landing could be made with low coral ledges.
 The rest of the island had coral cliffs up to 15 feet in height at the waters edge negating any assault plans.
 However, the Marine Corps asked the Seabees if they could come up with an idea to get over the coral.
 Commodore Paul J. Halloran (CEC) CB theater commander provided drawings of a conceptual landing ramp for the 18th and  121st CBs to fabricate.
  They mounted steel beams salvaged from Saipan's abandoned sugar mill on LVT-2s to create a portable assault ramps.
If they worked they would allow the Marines to outflank Tinian's prepared defenses.
 The Marine Generals was skeptical and ordered that the ramps be put through a 100 vehicle use tests.
  The Seabee creation was named a Doodlebug.
  It worked exactly as the Marines had hoped.
18th Marines was activated on 8 September 1942 and participated in actions at Tarawa, Saipan and Tinian. It was inactivated on 16 August 1944.
On Tinian there were two small beaches on the North end where an assault landing could be made with low coral ledges.  The rest of the island had coral cliffs up to 15 feet in height at the waters edge negating any assault plans.  However, the Marine Corps asked the Seabees if they could come up with an idea to get over the coral.  Commodore Paul J. Halloran (CEC) CB theater commander provided drawings of a conceptual landing ramp for the 18th and  121st CBs to fabricate.   They mounted steel beams salvaged from Saipan's abandoned sugar mill on LVT-2s to create a portable assault ramps. If they worked they would allow the Marines to outflank Tinian's prepared defenses.  The Marine Generals was skeptical and ordered that the ramps be put through a 100 vehicle use tests.   The Seabee creation was named a Doodlebug.   It worked exactly as the Marines had hoped.
18th Marine Regiment (United States)