In July 1943, at age 27, Metzger formed the 1st Armored Amphibian Battalion, and assumed command of the battalion a month later. He led the unit in battles in the Marshall Islands (Battle of Kwajalein), Guam (Battle of Guam), and Japan (Battle of Okinawa). He served as commanding officer of the battalion for 25 months. Military historians largely regard Metzger as one of the primary architects of the Marine Corps' armored vehicle force having led the first Landing Vehicle Tracked (LVT(A)) unit and written most of the armored amphibious doctrine for the vehicle's use in combat. He was also awarded the Legion of Merit with Combat "V" for his service during the Battle of Okinawa, where he volunteered to lead in the initial wave of combat on that Japanese island stronghold. In the weeks preceding the amphibious assault on Okinawa, he was promoted to lieutenant colonel.