In July 1942, Karch joined the 12th Marines, 3rd Marine Division as a battalion executive officer. From February until September 1943 he served as commanding officer, 1st Battalion, 14th Marines, and served for the remainder of the war as operations officer of the 14th Marines. Karch was awarded the Legion of Merit and the Bronze Star during the Roi-Namur, Saipan, Tinian, and Iwo Jima campaigns.