Why didn't the United States comply with the 1954 Geneva Peace Accord?

The basic reason that the United States did not comply
with the 1954 Geneva Accords was that they did not believe that their side would win the
elections.

According to the terms of the Accords, the
country of Vietnam was supposed to be reunited after elections.  Both the North and the
South would vote to see who would govern the whole, reunited country.  However, the
United States was worried about the likely outcome of the vote.  They knew that Ho Chi
Minh, the leader of the North, was very popular.  They also knew that Ngo Dinh Diem and
Bao Dai, the Southern leaders, were not popular.  Because of this, they realized that
the election would lead to a "bad" result.  This caused them to ignore the Accords and
simply allow the North and South to remain separate.