The Vietnam War was a long conflict that started after World War II and ended in 1975 with the reunification of Vietnam under a Communist government. It began as a war for Vietnamese independence against French colonial rule, then escalated with US involvement to prevent the spread of Communism in Southeast Asia according to the Domino Theory. Despite massive US bombing and increasing troop deployment, the Vietcong and North Vietnamese forces prevailed through guerrilla tactics and understanding of the local terrain. Growing domestic opposition in the US led to a withdrawal of troops and the eventual defeat of South Vietnam.