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World War I
World War I was also catastrophic but total casualties are much harder to calculate as deaths were not well documented. Some sources estimate that there were over 10 million military deaths plus civilian casualties, of which there are thought to be even more (so in total, the number of deaths is estimated at 20 million or more).
World War II
The largest and bloodiest war of the 20th century (and of all time) was World War II. The conflict, which lasted from 1939 to 1945, involved most of the planet. When it was finally over, between 62 and 78 million are estimated to have died. Of that enormous group, which represents about 3 percent of the entire world population at the time, the huge majority (over 50 million) were civilians.
Factoring in the deaths caused by the 1918 influenza epidemic, spread by returning soldiers at the end of World War I, this war's death total is much higher. The epidemic alone was responsible for at least 50 million deaths.
Russian Civil War
The third bloodiest war of the 20th century was the Russian Civil War. This war caused the death of an estimated 13.5 million people, almost 10% of the population—12 million civilians and 1.5 million soldiers. Unlike the two world wars, however, the Russian Civil War did not spread across Europe or beyond. Rather, it was a struggle for power following the Russian Revolution, and it pitted the Bolsheviks, headed by Lenin, against a coalition called the White Army.
Interestingly, the Russian Civil War was over 14 times deadlier than the American Civil War. By comparison, the latter was a much smaller war that resulted in 642,427 Union casualties and 483,026 Confederate casualties. However, the American Civil War, which began in 1861 and ended in 1865, was by far the deadliest war in history for the United States. The second deadliest in terms of American soldier fatality was World War II with a grand total of 416,800 military deaths.
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