US foreign policy stood adamentally, in opposition to Communism. It threatened their capitalist way of life, opposition with nuclear arms race, space race, what have you.
US, specifically enunciated by Eisenhower, stated (oversimplification) that once one country sways, there would be a contagion as other countries begin to fall to the "Reds". Russia -> China -> Indochina, then SE Asia, then Australia, New Zealand. So half the world would fall and threaten Western Democracy.
Interprutations by most historians see Ho not as a pure hearted communist, rather an opportunist who saw aspects of communism he could use in his nationalistic objective to free Vietnam from foreign rule (starting back with China in pre-Modern times, then French). In addition, Communism was willing to give Ho a hand in his aims where the US did not (minor point - he wasn't well known when he approached the US in any case, and when he was known, it was past the point of no return - i.e., he was a commie).
But American saw him as Red. Therefore, him wanting to unify Vietnam is equivalent to another domino falling to the infection of communism. And in an effort to stop the contagion, they decided on an active policy, meddled with France-Indochina politics, supported/funded an unpopular regime, sent in over half a million troops, then pulled out.
Long story short, they oversimplified, didn't see the complexities of Ho's situation.