How did vietnam end up being split into two?

in order for the the French to withdraw, and a nationwide election would be held
in 1956. However, in the South, a US-backed political force led by the Ngo
family called for independence and established the so-called Republic of Vietnam. The Democratic Rep. of Vietnam didn't agree with that and worked hard to unify the South. The Americans then came with military intervention. In 1973, US had to withdraw due to the Paris Accord. South VN then became difficult with lack of money and corruption. In 1975, Saigon fell to North Vietnam. In 1976, the two parts were reunified.