Why did us entered the war

US government viewed its involvement in the war as a way to prevent a communist take over of south vietnam . this was a part of a wider containment policy , with the stated aim of stopping the spread of communism . 
The US decided that it needed to hold the line against the communist , though and continued to send increasing amounts of war material and increasing numbers of military advisors to aid the capitalists of south vietnam . 
Gradually, the US got pulled into an all-out shooting war of its own with the North Vietnamese. First, military advisors were given permission to fire back if fired upon in 1959. By 1965, American combat units were being deployed. In April of 1969, an all-time high of over 543,000 US troops were in Vietnam.  A total of more than 58,000 US troops died in Vietnam, and over 150,000 were wounded.
US involvement in the war continued until 1975, shortly before the North Vietnamese captured the southern capital at Saigon.