The main assault on the island of Saipan began on June 15, 1944, and Tompkins landed on the same day. On June 17, Lieutenant Colonel Guy E. Tannyhill, commanding officer of the 1st Battalion, 29th Marines, which was attached to the 2nd Marine Division, was wounded by an enemy sniper and evacuated. Tompkins subsequently assumed command of the battalion on the same day and promptly reorganized the heavily depleted unit. His new battalion was located in a swamp area and pinned down by enemy snipers and gun emplacements. Tompkins made a hasty reconnaissance of the front lines and subsequently ordered his men and several tanks to the attack. During the attack, his troops captured vital objectives and overrun Japanese strongpoints according to plan.