Don't mess with the French.

"French Catholic missionaries had been proselytizing in parts of Vietnam with some success for two hundred years. But by the 1830s, a reaction had developed against them and a number were arrested and executed. In 1843, the French demanded that five condemned missionaries be released and sought to obtain the guarantee of rights of French nationals in Vietnam, much as European countries were doing in China at the time. Misjudging the strength of the French (as the Burmese had the British in India), the Vietnamese continued to harrass the resident French. After their demands were rejected, the French invaded Cochin China (present-day southern Vietnam) and took over by 1867. At the same time, they also turned their attention to Cambodia and forced the Thai, who then held it, to cede control to France. The French next addressed northern Vietnam and took Hanoi in 1882. The last piece of the French Empire in Southeast Asia was the territory of Laos, gained in the form of a protectorate in 1893, partly by getting Siam to cede control in the north. Together the three countries became the Colony of French Indochina."