 The Vietnam War , was a conflict between the Democratic Republic of Vietnam ( <unk> , <unk> or North Vietnam ) and the Republic of Vietnam ( <unk> or South Vietnam ) , which eventually involved their respective allies . In 1959 , the United States sent military advisors to train the Army of the Republic of Vietnam . By 1965 , there were 25 @,@ 000 military advisors in South Vietnam and on March 8 , 1965 , the United States Marines became the first US combat troops to land in South Vietnam , with a force of 3 @,@ 500 . 
