Domino effect
Communist State
North Vietnam, and southern communist guerillas living in South Vietnam (Viet Cong), (supported by the Soviet Union and Red China); verses the US, Australia, New Zealand, Philippines, Thailand, South Korea, and South Vietnam.
Vote cong
Viet Cong-
Viet Cong
The People's Republic of Viet Nam. (PAVN)
The Americans had a thing called the domino theory, that was if Vietnam became a communist country that all surrounding countries would as well. The Americans were really scared of Communism so they went to Vietnam to stop it.
North Vietnam (and its successor state Vietnam) are not democratic states. They are commonly considered "Communist States" and are, more specifically, Leninist One-Party States where the one party in charge is called the Communist Party. During the Second Indochinese War (called the Vietnam War in the US), the Communist Party of Vietnam was more "honestly" communist than it is today, when it has made a number of capitalist reforms. However, like China, the increase of economic reforms has not been complemented by a rise of political reforms.
They didn't stop it. The communist regime took over if you are unaware... Vietnam is still a communist country but it is not labelled that way to prevent eruption among the nations. There is a reason why Saigon is now called Ho Chi Minh City, after the leader of the communist party in Vietnam.
Vietnam
There was no "Vietnam" during the Vietnam War. Communist NORTH Vietnam was attacking non-communist SOUTH Vietnam, and the US was trying to drive the communists out of South Vietnam...trying to stop communist aggression. It's called the Vietnam War because it's a lot easier to say, and less confusing...than to say "South Vietnam War" or "North Vietnam War", so some people go so far as to say the war in Southeast Asia instead.
North Vietnam aided Viet Cong or the National Liberation Front (NLF) in a guerilla war against the anti-communist. The army of the North was called People's Army of Vietnam or North Vietnamese Army.