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China's Northern and Southern Dynasties period ended in 589 when the Sui Dynasty conquered the Chen Dynasty, but I think that China's reunification could still be delayed.

My suggestions for keeping China divided longer: First, Northern Qi survives as an independent state, instead of being conquered by the Northern Zhou in 577. This could happen with better leaders and if the Chen don't attack and weaken it. Second, Northern Zhou is weakened by internal struggles instead, perhaps between Emperor Xuan of Zhou and the man who would be Emperor Wen of Sui. Third, Chen is strengthened by the weakness of the Northern Zhou, keeping Sichuan at the very least and perhaps more bits of North China.

With a division of China into three powers, the era of Northern and Southern Dynasties would last longer, though it can't last forever. By 589, the Chen had maybe 2 million people. On the other hand, continued conflict between the Northern Qi and Northern Zhou might give Southern China a break. However, given the vast population imbalance, I might suggest that the Northern Qi would actually be in the best position to conquer the Chen by itself. In this case, we would see the conquest of the south before we get a reunification on east-west lines.

Thoughts?
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