Why was there a war in Vietnam?

Visual Perspective-understanding the Domino Theory and Containment

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The reason the United States got involved in the war is that they were afraid that Laos and Cambodia would fall to communism as well. 
According to the Domino Theory, since Laos and Cambodia border North and South Vietnam, they are at a high chance of eventually falling to communism if the South loses against the North.  
Containment is the foreign policy the United States followed during the Cold War. The ides of containment, by George F. Kennan, states that communism needed to be contained and isolated, to prevent the spread of communism.