太平天国 (Tàipíng Tīanguó), the Tàipíng Heavenly Kingdom, was a self-declared state (1850-1864) which rebelled against the Qīng Dynasty in the bloodiest civil war in the history of the world, leaving 20-30 million people dead - about 10 times that of the Vietnam War.
The war started after Hōng Xiùquán, born to a farming family in Guăngzhōu Province, had a vision that he was the brother of Jesus Christ. Combining religious preaching and increasing anti-Qīng sentiments (the government was losing big-time to foreign invaders in the Opium Wars), he was able to raise a giant army, take Nánjīng as his capital, renaming it and installing himself as Heavenly King.
After 14 years of fighting, Hōng Xiùquán died under mysterious circumstances and the Kingdom fell apart. His son was executed by slow slicing, which you don’t want to know what is.
History hasn’t been nice to Hōng Xiùquán, but he was nonetheless an anti-imperialist inspiration for both the Nationalist (1911) and Communist (1949) revolutions.
This battle barely suggests the massiveness of the war. Wikimedia Commons