At age 16, he enlisted in the Wisconsin Army National Guard from Helenville and served in the 32nd Infantry Division.
[1] On December 24, 1942, Gruennert was fighting the Japanese in the Battle of Buna-Gona in New Guinea as a sergeant with Company L of the 127th Infantry Regiment.
Near the village of Buna, he was second-in-command of a platoon tasked with advancing through Japanese positions to a beach 600 yards (550 m) ahead.
[2] Sergeant Gruennert's official Medal of Honor citation reads: For conspicuous gallantry and intrepidity in action above and beyond the call of duty.
His inspiring valor cleared the way for his platoon which was the first to attain the beach in this successful effort to split the enemy position.