Answer:
In its most basic form: after WWII, North Vietnam became communist while south Vietnam became democratic. Because the U.S. was strongly anti-communist and was afraid of a communist global takeover, they supported the south and deployed hundred-of-thousands of Americans to help the South Vietnamese troops. Communist rebels in the south, known as the Vietcong, staged mass, violent attacks against South Vietnamese democratic supporters as well as the American troops. After nearly 20 years of involvement, the U.S. pulled its troops out of Vietnam and the North unified the country under communist rule.