Antecedents
After the World War II all the countries were lying in ruins; two of them were obviously stronger: the United States of America and the Soviet Union. The others joined either the USA representing the democratic forces or the communist SU. The Cold War began, when opponents didn't meet each other, but supported local wars, like Korea, Cuba etc. and of course Vietnam. This was also a good terrain to try out newly developed weapons.
From the 19th century till 1954 Southeast Asia was French colony, including Vietnam, Laos and Cambodia. The French's situation weakened by the World War, where the Vichy-government concluded an alliance with Hitler. In 1945 Japanese troops invaded Indochina, but the International Court gave back the colony to France. Nevertheless, the country was so weak that eventually American taxpayers paid the 80 % of their war. The Vietnamese established parties for their independence that rebelled against the French power. The first party was the Viet Minh founded in 1941 by Ho Chi Minh. American president, Roosevelt didn't supported the colonization during the World War II, thus he didn't accept the French domination and sympathize with the Vietnamese, but his successor, Truman refused his predecessors guidelines. To prevent the spread of communism, the USA must interfere in Indochina, he said, for we must prevent the birth of the system based on Soviet pattern. The French, however, were decisively defeated at Dien Bien Phu in November, 1953; North Vietnam obtained its independence.
The Conference at Geneva started at the end of April, 1954 and was lasting for three months. Participants (USA, France, Vietnam, Laos, Cambodia, the UK, the Soviet Union and China) agreed that Southeast Asia was independent, concluded in a cease-fire and the withdrawal of French troops, and it was implemented. Vietnam was divided at the 17th latitude, forming a military border and in two years time, a referendum had to be held on the union of the country. The referendum wasn't held because according to American-friend leader of South, Ngo Dinh Diem the votes in North Vietnam wouldn't have been uncorrupted, therefore they boycotted it, so that it was frustrated... The division remained.