> military budget is greater than the sum total of all their neighbors is because of what

Because PRC has greater population and GDP than rest of region combined, of which she spends less than 2% on defense, less than most of her neighbours, less than what America wants NATO allies to spend on, and now less than JPs revised budget. All in all, too modest.

Same question, why the US has to spend 800 billion $ a year? worried about an invasion by Mexico?

That military budget got criticized by many Chinese. With a comparable GDP to the US, the CCP constantly refuses to further increase its military spending. As a result of such policy failure, its defense budget is just about 20% of the US one.

The question stands, irrespective of what legacy or current dysfunction the US is suffering from. And Taiwan Invasion is the logical answer. There is really no other reason. Just look at US struggling to even raise a single Division of troops in Europe to support Ukraine. If they cant do it in Europe, there is no way they are a serious threat as far away as Taiwan.