During this time, there were two major assumptions about the Soviet Union that informed American policy.
First, there was the assumption that the Soviet Union was in control of any and all communist movements anywhere in the world. This meant that the Viet Minh were seen not just as a Vietnamese independence movement run by communists but, instead, as a movement that was being controlled by the Soviet Union.
Second, there was the assumption that the Soviets wanted to spread communism around the world. This was the idea behind the "domino theory." This theory held that a communist Vietnam would lead to communism in other countries of Southeast Asia which would fall one by one like dominoes.
Because of these assumptions, US policymakers felt that it was extremely important to resist the communist insurgency in Vietnam.