On June 25, 1950, eight divisions of the North Korean People's Army, equipped with Soviet tanks, mobile artillery and supporting aircraft, crossed the 38th parallel and invaded the Republic of Korea. On June 27, the United Nations (U.N.) Security Council proclaimed the North Korean attack a breach of world peace and requested member nations to assist the Republic of Korea.

On June 30, President Harry S. Truman ordered a naval blockade of the Korean coast and authorized the Commander in Chief Far East, General of the Army Douglas A. MacArthur, to send U.S. ground troops into Korea. On July 2, MacArthur recommended that a Marine Corps regimental combat team be deployed to the Far East. The Joint Chiefs of Staff approved his request the following day.

On July 7, the 1st Provisional Marine Brigade was activated at Marine Corps Base Camp Pendleton, Calif. The core of the ground element was the 5th Marines, while Marine Aircraft Group 33 made up the air element of the brigade. Just five days after its activation, the 1st Provisional Marine Brigade, with a strength of over 6,500, sailed from San Diego en route to Pusan, Korea.