During the 1st Marine Regiment's early years, it was primarily employed as a combat force in the so-called Banana Wars, and in the Caribbean area.

World War II was the occasion for the next reactivation of the Regiment on 1 February 1941 at Culebra, Puerto Rico as part of the 1st Marine Division.

The 1st Marines stood at a low state of readiness at the beginning of the war, having just been reconstituted from cadre status; however, the regiment did possess very strong leadership at the higher levels.

[3] In June 1942, the 1st Marines set sail from San Francisco on board a mix of eight ships headed for the South Pacific.

[5] Some of the heaviest action the regiment saw on Guadalcanal took place on August 21, 1942, during the Battle of the Tenaru, which was the first Japanese counter-attack of the campaign.

The regiment was the first ashore at the Battle of Cape Gloucester on December 26, 1943; and continued fighting on the island, at such places as Suicide Creek and Ajar Ridge, until February 1944.

[10]During the Battle of Peleliu, the regiment was decimated by heavy artillery and accurate small arms fire in the vicinity of Bloody Nose Ridge.

[11] The 1st Marines fought on Peleliu for 10 days before being pulled off the lines after suffering 58% casualties and no longer being combat effective.

The presence of the 1st Marines was used as leverage by George C. Marshall in 1945–46 to attempt to moderate a settlement to the impending Chinese Civil War.

Following the termination of hostilities in July 1953, the regiment remained in Korea and acted as a defensive force against possible Communist attempts to resume the fighting.

There it stayed for the following ten years, except for a brief deployment to Guantanamo Bay, Cuba, and the Caribbean Sea, during the 1962 Cuban Missile Crisis.

The regiment remained deployed in South Vietnam for the next two and a half years, participating in numerous operations, both large and small.

On 28 June 1971, the last members of the regiment departed Da Nang to return to the United States at MCB Camp Pendleton.

From 1 to 11 May 1992, elements of the regiment deployed to perform riot control operations as part of the Joint Special Purpose Marine Air Ground Task Force Los Angeles.

[14][15][16] Subsequent to the collapse of the regime, the RCT conducted security and stability operations in Baghdad and Al Hillah until returning home throughout the summer of 2003.

Led by the 1st Marine Division, Operation Phantom Fury began with an assault north of the city, with four infantry battalions in the attack.

After twelve days of intense urban combat, 1st Marine Division had defeated the insurgents and successfully fought its way to the southern end of the city capturing the western half of Fallujah.