Up until 19th century, Western countries entered into the mainland Southeast Asia making several states in Southeast Asia became under their controls. For instance, France occupied Vietnam, Laos, and Cambodia, England occupied Myanmar, Netherland occupied Indonesia, and Portugal and Netherland occupied Malaysia.
Phitakthai Thepnok, Political analysis of countries in Asia. (Nakhon Ratchasima, Thailand: Jira Printing, 1999), p.7