Following the outbreak of the Korean War, 1/1 was reactivated at MCB Camp Pendleton on August 9, 1950. Later that month they deployed to Kobe, Japan and from there took part in the amphibious landing during the Battle of Inchon. In October, the Marines were withdrawn from the Seoul area and moved to the east coast of Korea landing at Wosnan in late October. From there 1st Battalion 1st Marines participated in the Battle of Chosin Reservoir. They pushed as far north as Koto-ri, spending much of the battle defending their perimeter in this vicinity.