Explain why the French were forced to leave Vietnam?

The French controlled Vietnam as part of French Indochina since the 1700s. In that time, the French colonial officials ran the area with their own interests in mind, treating the natives there like second-class citizens and mandating the use of French and the Roman Catholic Church.  During WWII, the area fell into the hands of imperial Japan, and the Vietnamese under Ho Chi Minh fought a guerrilla war against these new invaders from 1940 until the war's end in 1945.  The Vietnamese, hoping that the war would bring them more self-rule, were disappointed when the United Nations awarded the land back to the French government.   An organization that would later be called the Vietcong waged attacks against French garrisons in the country and France, severely weakened by WWII, could not fight back effectively even with American military aid.  The final battle in France's attempt to hold on to the colony was at Dien Ben Phu at which the Vietnamese resistance hauled howitzers up narrow mountain passes in order to shell a French fort.  This 1954 battle demonstrated to the French that the former colony was too hostile to own and France finally gave it up.  The United Nations divided the country into the North which received aid from the Soviet Union and the South which received American aid.  The two countries were supposed to be joined in an election in 1956, but the American-backed Ngo Diem did not allow this.