In 1921, the MCRDSD was formally commissioned and in 1923, it became the primary recruiting center for the west coast. During World War II, the flow of recruits into the base surged, with 18,000 recruits arriving in one month. In 1948, the base was formally named Marine Corps Recruit Depot San Diego and was home to the Recruit Training Regiment.
The 3d Battalion’s main mission is to train new United States Marine Corps recruits, specifically from west of the Mississippi River, but also from some areas east of the river, such as Wisconsin, Michigan, the Chicago metropolitan area and New Orleans. Recruit training includes a thirteen week process during which the recruit becomes cut off from the civilian world and must adapt to a Marine Corps lifestyle. During training, drill instructors train recruits in a wide variety of subjects including weapons training, Marine Corps Martial Arts Program, personal hygiene and cleanliness, close order drill, and Marine Corps history. The training emphasizes physical fitness, and recruits must attain a minimum standard of fitness to graduate by passing a Physical Fitness Test. Recruits must also meet minimum combat-oriented swimming qualifications, qualify in rifle marksmanship with the M16A4 service rifle, and pass a 54-hour simulated combat exercise known as "The Crucible".
Unlike training at Parris Island, recruits must leave the depot to conduct field training. Three weeks of the recruit’s training is spent at Edson Range aboard Marine Corps Base Camp Pendleton, where recruits fire on the rifle range, conduct field training, and undergo the Crucible. At the conclusion, recruits return to MCRD San Diego for graduation.