I thought this thread was a pretty good basic overview of the history around this conflict. One could go more in depth but it’s useful information to know as a starting point. Basically it boils down to bad borders drawn by European imperialists.

Also since it’s South East Asia you can bet the US is somehow involved in stirring up any conflict that breaks out, since they are the only ones who stand to gain from destabilizing the region. They have their fingers pretty deep in both countries.