Arguably, the US civil war wasn't strictly necessary. If it weren't for the slavery issue, I think it's would have been better for the northern states to just let the south secede. Even with the slavery issue, and if they had not bothered fighting a war to keep those states in the union, it's arguable that the south would have failed economically before long anyway, especially if the north had instituted sanctions.

As for comparing democracies to China, the democracies of the EU are lucky China isn't next door to them. They're already completely unable to defend themselves against Russia, which is also expansionist and aggressive, but at least doesn't have China's industrial power. If the EU were next to China, they'd be completely defenseless against Chinese expansionism.