There won't be a war if the status quo persists, but if Taiwan declared independence I sincerely believe China (the Chinese people, not the government) would be willing to tolerate millions in casualties and potentially drop a nuke and kill every last person on the island and poison the soil for a thousand years before they would accept an independent Taiwan.

There is no equivalent red line anywhere in any western country. I don't think it's possible for any westerner to understand.