The Vietnam War started in 3-4 mirco phases-stages:
It began shortly after the end of World War II in 1945 with a war for independence from centuries old French colonial rule.
In 1954, the French withdrew its colonial forces from Vietnam. Although it was a victory for Vietnam, it was, in retrospect, a short-lived victory.
Almost immediately after the French departure from Vietnam-(and greater Indochina), the second phase of the war consisted of the Vietnamese turning on each other thereby leading to a Civil War. The Communists, led and inspired by Ho Chi Minh, were widespread in North Vietnam-(with growing support from the Soviet Union and neighboring China), while South Vietnam (initially led by Diem) remained culturally Buddhist and Catholic with growing Advisory support from the United States, specifically from the Eisenhower and Kennedy administrations. The Vietnam war raged for 10 years whereby Vietnamese were fighting Vietnamese.
However, The 10 yearlong Vietnamese Civil War transitioned into a third phase, whereby the United States (under President Lyndon Johnson) increased its military presence with mass troop deployments-(both enlistees and draftees),shorty after the passage of 1964 Gulf of Tonkin Congressional Resolution. For the next 8 plus years, what is called, "The Vietnam War" occurred.
It was also a wider war that expanded into nearby Cambodia in 1970-(representing almost a fourth beginning micro-phase/stage and in actuality, creating a greater war in Indochina, which was not limited to Vietnam alone).