At what point in its history was southeast asia a buffer zone?

The correct story to this open question is the following.

The point in its history when Southeast Asia was a buffer zone was during European colonization.

In the history of world foreign affairs, a buffer zone is considered to be that country that is in the middle of two nations that conflict. Both countries in conflict respect that "buffer zone or buffer country" and do not attack it or invade because sometimes it served as an intermediary to resolve a conflict. For instance, that was the case of Thailand during the European colonization. France and Great Britain tried to colonize Thailand with no major success.

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