Why did Americans help the french against the viet minh?

originally the Americans were on the side of the Viet Minh. during world war two the Americans supplied the viet minh with aid and money to support their war effort against the french and the Japanese, because they wanted the Japanese defeated and did not like the idea of the old colonial powers having empire again. the American's did not care that at the heart of it Ho Chi Minh was a communist.

however after world war two finished this all changed. the Americans were in the midst of the cold war with communist russia and they did not want any more countries of the world to become communist, as most of eastern Europe was already. the American president, Truman, did not want Another Country to become communist, and so created the Truman doctrine with the policy of containment. The Americans swapped sides and started aiding the french, claiming that they were fighting for a democratic country, because they did not want to see the communists in power.

this ended in the battle of Dien Bien Phu when the french lost dramatically and the Geneva convention agreed that Vietnam shoudl be split into two, the north as communist and the south as demoscratic.

so really the general answer summed up is: because the Americans were afraid of the spread of communism and wanted to stop Vietnam becoming communist at all costs.