How were the countries of Southeast Asia altered by the arrival of the Europeans?
The coming of the Europeans had little immediate impact on the countries of mainland Southeast Asia. The Dutch were able to colonize the Indonesian archipelago, but countries on the mainland were not colonized.
European countries did establish trading posts on the Southeast Asian mainland. They also tried to use these posts as places from which to do missionary work. Because the various European countries wanted preferential trading relations with the Asian countries, they tended to get involved by backing one side or the other in internal conflicts. However, this did not translate to strong control.
Eventually, most European nations gave up their trading posts as the efforts were not worth the gains. In this way, Southeast Asia remained relatively unaffected by European expansion through the end of the 17th century.