After World War Two, the Soviet Union was very large and had a lot of influence over other countries. This is one of the reasons that China became Communist. When Vietnamese began to rebel against the French, the Soviets supported them, and North Vietnam became Communist. This created a lot of tensions between North and South Vietnam.
Operation Rolling Thunder
In 1965, two U.S. ships in the Gulf on Tonkin reported being fired upon by North Vietnamese patrol boats. This caused America to go to war with North Vietnam, despite the fact that there were major doubts about the veracity of the incidents.
In 1968, the Charlie Company, under the direct command of William Calley, opened fire on hundreds of unarmed Vietnamese.
End of the Vietnam War
Operation Rolling Thunder was a gradual and sustained bombing campaign against the North Vietnamese by the U.S.
In 1968, the North Vietnamese launched an attack against over 100 South Vietnamese Cities with a force of about 85,000 on the first day of the lunar New Year.
After complete U.S. withdraw from Vietnam by 1973, by 1975, the whole of Vietnam was united under the Communist North.