8. World War I
Death Toll: 15 to 21 million; Period: 28 July 1914 to 11 November 1918; Warring Parties: Allied Powers and Central Powers
The war started with the assassination of Archduke Franz Ferdinand of Austria who was the heir to the throne of Austria-Hungary. It set into motion a series of events which led to the rest of European countries choosing sides as Austria-Hungary declared war over the Kingdom of Serbia. The Central Powers included Germany, Austria-Hungary, the Ottoman Empire and Bulgaria. The Allied Powers consisted of a larger number of including the British Empire, the Russian Empire, France, and Serbia, later joined by Italy, Japan and the United States.
The war completely changed the political map of Europe with some nations gaining independence, others newly created and some empires completely gone. The economic depression and other issues brought on by the First World War soon led to discontent in Germany, eventually leading to the beginning of Nazism and the Second World War.(source)
7. An Lushan Rebellion or An Shi Rebellion
Death Toll: 13 to 36 million; Period: 16 December 755 to 17 February 763; Warring Parties: Yan Dynasty and Tang Dynasty
The rebellion started with the establishing of Yan dynasty when general An Lushan declared himself the emperor in Northern China. After 8 years of Yan fell but the rebellion continued after An Lushan’s death by his son and his deputy. It took three generations of Tang emperors to completely eradicate the rebellion. By the end of it, Tang dynasty suffered serious losses, including the Western Regions, and subsequently weakened.(source)
6. Manchu Conquest of China or Qing Conquest of the Ming
Death Toll: 25 million; Period: 1618 to 1683; Warring Parties: Qing Dynasty and Ming Dynasty
In early 1600s the Qing dynasty, established by Manchus, was dominated by Ming dynasty. A leader of Qing dynasty, Nurhaci, demanded the Ming dynasty pay him tribute for the Seven Grievances caused by them to one of the Manchu clans. However, the Ming dynasty didn’t take it too well, which led to war.
Ming dynasty was facing several other internal problems at that time – peasant rebellions, fall of Beijing, severe financial troubles and strained relations between royalty and military. In 1661, a new emperor ascended the throne of Qing dynasty and launched Great Clearance defeating the rest of resistance from Ming dynasty.(source)
5. European Colonization of Americas
Death Toll: 8.4 to 138 million; Period: 1492 to ; Warring Parties: Europeans and Native Americans
After Christopher Columbus’ discovery of the Americas in the 15th century, Spain and Portugal began their conquests and colonization. British, Danish, Dutch, Scottish, French, Russian and other kingdoms were the others that colonized the continent. The first European explorations, however, happened as early as 10th century when Vikings came and settled briefly in Canada.
There were many factors that contributed to the death toll during colonization. Along with the brutality of the European conquerors many diseases spread during the Columbian Exchange resulted in such colossal destruction of indigenous population.(1 , 2)