The Three Kingdoms Period in China is the one that comes to mind for me. After the loss of power by the Han dynasty circa 220, China was divided into three factions (Wei, Shu, and Wu). These three factions all fought against each other for a period of around 60 years, which marked the bloodiest period of time in China's history. A census before the fall of Han estimated the population at around 50 million, while a census post Three Kingdoms during the Jin dynasty estimated it at only about 16 million.

Whether the three kingdoms were really fighting ferociously against each other all at once, or whether it was more of a 1984-esque scenario, I'm less certain. However, I do know that during this period, each faction was at war with every other faction at some point during the period.

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