The beginning of modern strife in Vietnam was due to a communist revolution against the French, who had colonized much of Southeast Asia. Ho Chi Minh led the uprising but fled after the French began to execute dissenters.
The Japanese took over Vietnam during WWII, but the French returned to regain it afterwards. The Vietnamese joined forces to drive them out. Having defeated the French, Vietnam split into a communist nation (North) and a dictatorship (South).
The US decided to support the South in order to prevent the dominoes of communism fom furthering. However, the US army was ill-suited to fighting on land in Vietnam. In addition, there was little to no support on the home front, where nobody wanted to fight a war half the world away.
After the Vietnamization of the war, the US pulled out entirely. Within the next few years, South Vietnam was entirely taken over by the North.