It's hard to pinpoint one specific start of the Vietnam War, as the conflict escalated gradually across three decades. Vietnam was a French colony for a long time, and fighting first began in the 1950s when a communist insurgency started rebelling against the French occupation. After the French suffered a major military loss at the Battle of Dien Bien Phu, they chose to give up.
At this point, the United States started sending in military forces as part of its policy of containment, which sought to restrict the expansion of communism around the world. American forces went to Vietnam first as advisers to the South Vietnamese government (which was fighting against the communists in the north of Vietnam), and later as combat troops. By the late 1960s, there were hundreds of thousands of American soldiers fighting in Vietnam.
