Amazing info on an intriguing question, thanks!

Now I’m intriguided by the question of why Western Europe wasn’t plagued by the same issues.

To an extent, it was. France famously lagged behind because of land wars in Europe.

But what helped, I think, is the natural borders such as mountains, and all the neighbors being settled people.

Even the sea-based threat that Taiwan was was caused by the Qing’s arrival, which was caused by a land-based threat (the Manchus).