The United States lost the will to continue fighting. The Vietnamese, who were defending their own country, were willing to fight forever. The North Vietnamese saw no distinction between North and South Vietnam. Ed Vietnam War Veteran 1968-69
The policy of Soviet (Communist) containment meant that the communists would not be allowed to take over South Vietnam. We would destroy the communists in South Vietnam and contain them to NORTH Vietnam.
South Vietnam
North Vietnam/VC vs allies
North
Air war North Vietnam. Ground war South Vietnam.
The US wanted N. Vietnam to stop fighting in S. Vietnam.
South Vietnam's surrender to North Vietnam is what ended the Vietnam War. The North Vietnamese had the goal of Vietnamese Unification and the surrender of South Vietnam in 1975 allowed them to achieve that objective.
North Vietnamese communists
North Vietnamese troops would be allowed to stay in South Vietnam.
Not much of one: curtail incursion, support the South Vietnamese, bomb the supply routes and continue fighting until the North abandoned the battlefield.
North Vietnamese troops would be allowed to stay in South Vietnam.