In early 1942, Schrier joined the 2nd Marine Raider Battalion at Camp Elliott, San Diego, being promoted to platoon sergeant in April 1942. In June 1942, he was part of two 2nd Raider Battalion companies that were sent to Midway Island to bolster the garrison there. He participated in the Battle of Guadalcanal and found himself taking part in the 2nd Marine Raider Battalion's epic "Long Patrol" behind enemy lines from November to December 1942. During this action, he distinguished himself by leading part of his cut off company to safety after his company commander erroneously led them into a hostile situation. In early 1943, he was promoted to second lieutenant in the field. Subsequently, he was detached to other duties within the Raider organization, such as observation and reconnaissance on enemy-held islands before larger units made assault landings. He was awarded the Legion of Merit for his work at Vangunu Island during the New Georgia Campaign in June 1943. He also served at Bougainville in support of military actions there. In February 1944, after the Marine Raiders were disbanded, he returned to the United States to become an infantry instructor at Marine Corps Base Camp Pendleton. He was assigned next to be the executive officer of E Company, 2nd Battalion, 28th Marine Regiment, 5th Marine Division. In September, the 5th Division left Camp Pendleton for Hawaii, for further training. In January 1945, the 5th Division left Hawaii for the assault and capture of Iwo Jima.