Dear Student,
US was worried about the alliance of NLF and Minh's government because:
a. The US feared that a communist government would come to power in Vietnam after the National Liberation Front formed a coalition with the Ho Chi Minh government in the north, against Ngo Dinh Diems regime.
b. US policy-planners feared a spread of communism to other countries in the area, that is it would have a domino effect.
c. US entry into the Vietnam war was the outcome of humiliating defeat of the France, a European country, at the hands of a small Asian country Vietnam.
d. The unification of the North and south Vietnam which was in violation of Geneva convention was closely watched by US which feared the rising power of the communist.
e. Ideological differences that prevailed with US representing capitalist bloc and Vietnam communist ideology
Regards