12. Napoleonic Wars
Death Toll: 3.5 to 7 million; Period: 18 May 1803 to 20 November 1815; Warring Parties: France, led by Napoleon I and Many European Powers
Following the French Revolution, there was an increase in unprecedented compulsory mass enlisting of people into military service. With Napoleon conquering Europe, losing only 7 out of 60 battles he fought, the French Empire soon became very powerful. The confidence it gave him led to the conquest of Russia which failed because of harsh Russian winter and depleted resources.
A few other successive defeats caused the French to distrust his ideas and to finally exile him to the island of Saint Helena. The Napoleonic wars resulted in the break down the Holy Roman Empire and changes the political conditions of Germany and Italy.(source)
11. Thirty Years War
Death Toll: 3 to 11.5 million; Period: May 1618 to May 1648; Warring Parties:Habsburg States and Anti-Habsburg States
Ferdinand II, the newly elected Holy Roman Emperor, began to impose Roman Catholicism on his people to bring uniformity of religion. But the protestant population in the north of Europe felt this was against their rights to choose and formed Protestant Union. Also, the Bohemian protestants in Habsburg, Austria, elected Frederick V as their monarch instead which angered the Roman Catholic states in the south.
After that the Holy Roman Empire crushed the rebellion which led to majority of the countries in England taking sides. With soldiers looting and the famine war brought millions have died and entire regions were devastated.(source)
10. Russian Civil War
Death Toll: 5 to 9 million; Period: 7 November 1917 to June 16, 1923; Warring Parties: Red Army, White Army, Green Army and Others
After the Russian Revolution in 1917 and the abdication of Tsar Nicholas II, a rivalry broke out between many groups that wanted to lead the future Russia. The majority of casualties were civilians and it was considered the greatest catastrophe in European history.During this time several parts of the Russian Empire sought independence and were established as sovereign states.(source)
9. Chinese Civil War
Death Toll: 8 million; Period: 1 August 1927 to 22 December 1936 and 31 March 1946 to 1 May 1950; Warring Parties: Nationalists and Communists of China
The Chinese Civil War is a result of clash between the supporters of two political ideologies – Kuomintang’s (KMT) nationalism under the leadership of Chiang Kai-shek and Communist Party of China’s (CPC) communism under the leadership of Mao Zedong. The conflict resulted in establishing two separate governing states, the Republic of China which is functional in Taiwan and other outlying islands of China, and People’s Republic of China in mainland China.
The two sides, however, ceased fighting for a brief period between 1937 and 1945 to fight Japanese Invasion. Though the battles stopped by 1950, there has never been a peace treaty signed by the two.(source)