Events at what colonial fortress city marked the end of foreign rule of Vietnam?

Dien Bien Phu.

The fortress city was where French colonial power ultimately fell to Vietnamese communist forces in 1954. This marked the end of France's control in the region.

In America, we remember the Vietnam War as a long, protracted struggle in which the United States military was not able to succeed over motivated local forces. What we often don't remember is that the path to war in Vietnam began with revolution against colonial government by the French. The US became involved because we had made a commitment to work against the spread of communism.