"Our purpose in Vietnam is to prevent the success of aggression. It is not conquest, it is not empire, it is not foreign bases, it is not domination. It is, simply put, just to prevent the forceful conquest of
South Vietnam by North Vietnam."

Lyndon B Johnson, 36th US President

 

The Causes of the Vietnam War: Andrew J. Rotter

http://www.english.uiuc.edu/maps/vietnam/causes.htm