As I read Southeast Asia history, I found some of the nations were kind of constructed based on who colonized the territory before rather than evolved from older native entities, e.g. Indonesia, Malaysia, Singapore, Timor Leste, Papua New Guinea, etc. What was the effect of colonialism to the national identity? Why did the locals ditch the previous "local identity" for new "national identity", given the national identity was sprung from foreign colonialization? How did the colonizers react to the nationalists? "Wow, we accidentally created a new nation"? How long it took until all locals in the area identified with the "national identity"? Did they found any conflict with their previous identity? I suppose it was not one-night process.
Great questions! This one's going to be a multi-part answer, and I'm quite looking forward to what people with expertise in other countries or topics have to say. (1/4)
Every modern state in Southeast Asia reflects colonial intervention in some form. This is not to privilege colonial decisions and strategies, and to say that local communities did not have any influence or agency on the matter. How national identities were negotiated, constructed, interpreted and strategically deployed over time are factors that are just as important a factor. There’s going to be significant variation amongst the countries you’ve mentioned. Something beginning as a British colonial port with a tiny indigenous population like Singapore will lead to very different demographic compositions from an archipelago like Indonesia, while the circumstances of independence also matter. A relatively peaceful decolonization after a prolonged and forceful anti-communist campaign, as was the case in Malaysia, will shape national identity in quite a different way from a protracted liberation conflict enmeshing a neighbouring occupier and the international community, like in Timor Leste. I’ll accept your question’s premise and identification of some states as “new countries”, but also point out that Siam/Thailand and Vietnam, although claiming long and continuous histories (as contiguous entities, too), cannot be understood without reference to colonial encounters and various forms of colonization too. I’ll focus my answer on Singapore, a country I’m more familiar with, but engage with other nations too.
I’ll make the obligatory Benedict Anderson reference/aphorism and point out that nations are imagined political communities – it’s useful to bear in mind that he was an Indonesian specialist. In essence, individuals within relate to each other and to a greater community through symbolic, constructed understandings and associations. Partiality is inherent in the development of these identities. Acts of remembrance and representation, for instance, produce evolving narratives that make sense of the past. What might be more useful than thinking of separate, discrete and oppositional “local” and “national identities” is to think of a plurality of identities. It certainly wasn’t a one-night process as you point out, and the conflict between colonial authorities and nationalists was often very messy. A useful concept to bring in here is that of ‘historical memory’, something that “stresses the presence of the past in everyday life” while assimilating the past into the present (Richards). In other words, how and why did colonial and post-colonial societies interpret the past in a way that came to enable national identities? And what if anything, are the national identities we’re trying to explain?