What guided the US policy position with respect to the Geneva Conference was the American desire to make sure that South Vietnam (or the country of Vietnam as a whole) did not fall under communist control.
The Geneva Conference called for an election for a leader who would govern the entire country. Vietnam would be reunited under this leader. However, the US was quite sure that Ho Chi Minh would win any nationwide election in Vietnam. It was for this reason that the US did not sign the Geneva Accords. It is also why they never allowed the election to take place.