On June 25, 1950, the North Koreans (NK) invaded the South
Striking in overwhelming force, without warning, they crushed the unprepared Republic of Korea (ROK) army. The NK were only contained by the entry of the United States, quickly supported by the United Nations.
With the North Korean invasion hardly a week old, and their armies running amok down the Peninsula, General MacArthur began planning an amphibious assault to retake the communications center at Seoul. This would trap the main NK combat forces, and permit us to destroy them at leisure.
President Truman, guided by his Cabinet and the JCS and a strong personal dislike of the USMC, had cut the Fleet Marine Force to 34,000 officers and men, giving a ground fighting strength of only six infantry battalions, and a total Corps strength of 74,279 officers and men. Eliminated were the two Marine divisions which would surely have enabled Eighth Army to meet and defeat the In Min Gun in the Pusan Perimeter. Corps strength was so reduced that the 3 battalions available for a provisional brigade only had two companies each.
Originally selected by MacArthur for the amphibious operation, early Eighth Army defeats made the need for reliable infantry in the perimeter paramount. 5th Marines formed the 1st Provisional Marine Brigade and landed at Pusan 8/3, serving with distinction at the Naktong Bulge, well supported by their M26 Pershings and Marine Corsairs. By "distinction", the 5th Marines typically won their engagements with the NK forces whereas Army units did not, and half the Army casualties were usually MIAs whereas the 5th usually had none. On 9/12, the 5th mounted out from Pusan to help form the rest of 1st Mar Div and X Corps at sea.
1st Marines were activated in August, and staged in Japan. 1st joined the Inchon invasion in the afternoon of the first day, assaulting Blue beach.
7th Marines were activated around 9/1, staged at Kobe around 9/9, and on 9/21 were landed at Inchon to join 1st Mar Div.