South East Asia is taken in this history to include the countries of the Asian mainland south of China, from Burma in the west to Vietnam in the east and the islands from Sumatra in the west to the Philippines and New Guinea in the east.
It does not include Taiwan (Formosa), whose history seems to be more naturally part of that of China. But it does include Hong Kong and Macao, the British and Portuguese possessions on the south China coast, as their history is bound up with that of South East Asia rather than with that of China.
With so many different countries being covered, the history of any one country is necessarily fragmented. The following index makes it possible to read the history of each, if so desired, more or less consecutively.
For more explanation, you can download the document.
A Short History of South East Asia