Answer: after the WW II last cycle of decolonization process was initiated. Logically, Soviet Communism (expansionism) was involved in this process. French tried to 1) keep their power in Vietnam and 2) contain pressure of Vietnamese Communists. But were unable to do so. The French withdraw and the Americans took over all the containment issue in Vietnam. Expansion of Communism was at stake.
Explanation: there ware many similar (even though not exactly the same) situations in other parts of the world. Vietnam became famous because of the failure of the USA. Danger of expansion of Communism took place in Kongo, Angola, in Portugal (after 1974) and elsewhere.