The Vietnam wars did not begin with the French return to Vietnam at the end of the Second World War, but with the British intervention that preceded it. The story of this intervention ought to be more widely known.
The French Collapse
After the fall of France in the summer of 1940, the French administration in Indochina pledged its loyalty to the pro-Axis Vichy regime. This did not, however, save it from increasing pressure from the Japanese, who were determined to bring the colony under their control. To this end, the Japanese initiated serious border clashes in September 1940 and threatened to bomb Hanoi. This threat brought about a speedy French capitulation to their demands. The French accepted the establishment of Japanese military bases throughout Indochina. In return, they were left to rule over their Vietnamese subjects much as before.
The French humiliation at Japanese hands emboldened the Vietnamese nationalists. At the end of September, the first of a series of Communist-led insurrections broke out, and was ruthlessly suppressed by the French, with over 8,000 killed. According to Joseph Buttinger:
Airplanes and artillery razed entire villages and small towns...In view of the desperate mood of the French, it is surprising that any prisoners were taken But the treatment meted out to some was atrocious. They were brought to Saigon by night on river boats, and they could be seen standing in long rows in the harbour under glaring searchlights, strung together by wires pushed through the palms of their hands. 
For the Allies at war with Japan, the situation in Indochina posed considerable difficulty. as far as the British were concerned, the objective was to drive out the Japanese and return Vietnam to the Free French under General Charles de Gaulle as part of a colonial restoration throughout Southeast Asia. The American attitude, however, was different.
Astonishing as it might seem in view of later developments, the United States was at that time hostile to colonial restoration in general, and to the reassertion of French colonial rule over Indochina in particular. president Franklin Roosevelt had on numerous occasions made clear his lack of sympathy for the French and their empire. "Indochina should not go back to France," he had told secretary of State Cordell Hull in mid-October 1944. "France has had the country--thirty million inhabitants--for nearly one hundred years, and the people are worse off than they were at the beginning."  Instead, Roosevelt proposed the replacement of French rule by some sort of trusteeship that would prepare the way for independence. These sentiments had repercussions in the theater of war, where Americans not only refused to cooperate with the French, but actually provided assistance through the Office of Strategic Services (OSS) to the communist-led Vietminh.
In March 1945 the Japanese responded to their rapidly deteriorating military position elsewhere by staging a coup d'etat in Indochina, finally eliminating what remained of French power there. On March 11 Japan proclaimed Vietnam independent of France and set up a puppet government under the Emperor Bao Dai.
Most French army units were taken by surprise by the coup and were quickly disarmed and imprisoned. But some offered resistance and tried to fight their way to China. They appealed to the Allies for assistance. The British were eager to help, but the americans, although much better placed, refused. This situation brought to a head a continuing jurisdictional conflict between the British and U.S. military commands in the area.  Both the British and the French were outraged by the American attitude, and some still are. one eminent British historian has argued that Roosevelt's refusal to help the French at that time "can only be compared with that of the Russians toward the Warsaw rising the year before." In his dying days, according to this account, Roosevelt had brought "dishonour . …