4. Mongol Conquests and Invasions
Death Toll: 30 to 40 million; Period: 1206 to 1337; Warring Parties: Mongol Empire and Surrounding Areas
Mongol conquests are the second largest conquests in history, the first being the British, and the first largest contiguous land empire. Throughout the 13th century the Mongolian Empire spread to almost all of Asia, except for southern most parts, and to Eastern Europe. The conquests started with the rise of Genghis Khan as the emperor. The first half of the century saw the empire spread to Central, Western, Eastern and Southeastern Asia. Though after that the empire started to become fragmented, they raided Russian states. The empire finally broke into separate entities by end of 13th century.(1, 2)
3. Three Kingdoms War
Death Toll: 36 to 40 million; Period: 184 AD to 280 AD; Warring Parties: The Chinese States of Wei, Shu and Wu
The actual period of the Three Kingdoms, though, is between 220 AD and 280 AD, before which the states were under Han dynasty and under Jin dynasty after. The division into tripartite happened after 220 AD after the fall of Han dynasty.
The Three Kingdoms is considered one of the deadliest war period in history and one of the bloodiest in Chinese history. Since around 184 AD the areas, which were yet to be divided, engaged in “chaotic infighting among warlords”. From 220 AD it turned into a more organized militarily stable rivalry as they were divided into three clear rivals. In 263 AD Wei conquered Shu and was later overthrown by Jin dynasty in 265 AD. Jin also conquered Wu in 280 AD.(source)
2. Taiping Rebellion
Death Toll: 20 to 100 million; Period: December 1850 to August 1864; Warring Parties: Manchu-led Qing Dynasty and Christian Millenarians
In a town called Jintian of Guangxi province, a man called Hong Xiuquan declared himself the younger brother of Jesus Christ and the Heavenly King of the Taiping Heavenly Kingdom. His God Worshiping Society was attacked by the local officials of Qing Dynasty as part of a campaign against the Christian sect. This soon escalated to civil war as the society, also referred to as Taiping, started marching north, fighting and occupying some of the wealthiest areas of the province, including the capital Beijing.
A local army called Xiang Army fought the Taipings on behalf of the Qing imperial army and succeeded in weakening them by 1856. Finally in 1864, Hong’s son, leading remaining forces of the Taipings, was captured essentially putting a close to the rebellion. The scattered forces were completely annihilated by 1866.(source)
1. World War II
Death Toll: 65 to 85 million; Period: 1 September 1939 to 2 September 1945; Warring Parties: The Allies and the Axis
The Second World War is the deadliest of all wars so far, resulting in the highest number deaths and involving majority of the countries in the world. It is officially said to have started when Germany invaded Poland and later declared war on France and United Kingdom. Germany and Soviet Union took over Finland, Romania and Baltic states along with Poland.
In Asia, Japan was at war with China by that time and was looking to dominate Asia and the Pacific. The aggressive advances of Germany, Italy and Japan, together forming the Axis, were unstoppable. To fight them Great Britain, France and Poland, and later the British Commonwealth nations, formed the Allies. The Soviet Union and United States joined after being attacked by Germany and Japan respectively.(source)