The 1st Battalion sustained its first major casualties of the war on 21 December 1965 when the enemy ambushed the command group of Company B as the company was moving out of Bien Hoa on routine patrol. On 25 August 1966 during Operation Amarillo a patrol from Company C, 1st Battalion was ambushed after stumbling into a Viet Cong base camp, losing 6 men killed of the 15-man patrol, total US losses in the operation were 41 killed, 45 Viet Cong bodies were found, while later intelligence indicated that Viet Cong losses were 171 men killed. The 2nd Battalion fought its first major battles at Ap Bau Bang on 12 November 1965 and Ap Nha Mat on 5 December 1965. Heavy losses were suffered at Ap Nha Mat and three soldiers are still listed as missing.