Short answers, pulled from the story.
Estimates of the Taiping Rebellion's death toll range from 20 to 30 million, representing 5 to 10 percent of China's population at the time. Some analysts have placed the figure as high as 100 million, approaching one quarter of the Chinese population, which would make it the deadliest civil war in human history. Most deaths were attributed to plague and famine rather than direct combat.
Hong Xiuquan was a Hakka man from Guangdong who led the Taiping Rebellion after a series of visions convinced him he was the younger brother of Jesus Christ and the second son of God. He failed the imperial civil service examinations four times, the last failure occurring in 1843, after which he read Christian pamphlets that he believed explained his earlier dreams. He founded the Taiping Heavenly Kingdom and declared himself its Heavenly King on the 11th of January 1851.
The Taiping Rebellion began with a major battle at Jintian, Guangxi in early January 1851 and the fall of Taiping-controlled Nanjing came in July 1864, ending the main phase of the conflict after 14 years. The last confirmed Taiping army, led by Li Fuzhong, was destroyed in August 1871 in the border region of Hunan, Guizhou, and Guangxi.
At its peak, the Taiping Heavenly Kingdom ruled over a population of nearly 30 million people. The rebels captured Nanjing, which they renamed Tianjing, in March 1853 and made it their capital. Their armies occupied and fought across much of the mid- and lower Yangtze valley, including major cities such as Wuchang, Anqing, Hangzhou, Changzhou, and Suzhou.
Western powers officially remained neutral but intervened on the Qing side in limited ways. The Ever Victorious Army, a Qing force of Chinese soldiers commanded by Western officers including Frederick Townsend Ward and Charles George Gordon, was instrumental in repelling Taiping attacks on Shanghai beginning in 1861. The Royal Navy also imposed a naval blockade that helped Zeng Guofan's forces capture Anqing in 1861. American and English smugglers sold weapons to both sides, briefly leading to a skirmish known as the Battle of Muddy Flat in which British and American forces fought alongside the Taiping.
The 14-year civil war severely weakened the Qing dynasty's economic and political viability and accelerated the decentralization of power from Beijing to provincial gentry-led armies. These provincial forces, which historian Franz H. Michael argued evolved into warlord armies, contributed to the loss of central control after the Republic of China was established. The Warlord Era began in earnest in 1912.