Well, the Geneva Conference of 1954 basically put Vietnam independent of outside rule, including French and whatnot, and separated the country into communist North and non-communist South by the 17th parallel. They were meant to hold elections that would unite the country, but the South blocked the elections for fear of communist rule. From there, conflicts began to pick up -- the Viet Mihn began taking military action against South Vietnam, and the Vietcong, communist sympathizers in South Vietnam, used guerrilla warfare from within South Vietnam.
America began giving South Vietnam military training during Eisenhower's years because the South Vietnamese army was not really that great and they requested help, but we didn't actually send any troops to help in military conflict until Kennedy's presidency. From there, the conflict escalated until Vietnamization was set into motion.
The conflict was between the Vietnamese, and the Vietnamese were meant to fight. They were being heavily influenced from the U.S. and the U.S.S.R., but they were the ones fighting, and they were not "innocent victims." The only innocent ones were civilians, but that goes for any military conflict.
They quite possibly would have fallen to communism anyways. After Vietnam became communist, Laos and Cambodia became communist as well.
And no, atrocities shouldn't be forgiven, but they should be understood. We need to learn from that.