Final answer:
The US ally in the Vietnam War was the Army of the Republic of Vietnam (ARVN), which fought alongside US forces against the North Vietnamese Army and the Viet Cong.
Step-by-step explanation:
The group that was a US ally in the Vietnam War was the Army of the Republic of Vietnam (ARVN). The ARVN was the military component of the regime of South Vietnam. During the Vietnam War, the United States provided military and economic support to South Vietnam, attempting to bolster the ARVN to prevent the spread of Communism from the North. As an ally, the ARVN fought alongside US forces against both the North Vietnamese Army (PAVN) and the Viet Cong (VC), although their effectiveness was often questioned by their American counterparts. Notably, the Viet Cong were supported by the North and utilized guerrilla warfare tactics which posed significant challenges to US and ARVN forces. The US's involvement in the Vietnam War was also part of a larger Cold War strategy to oppose Communism wherever it emerged, which explains their alliance with South Vietnam.