The Major Wars and Conflicts of the 20th Century

Most Deadly and Significant Wars in the 1900s

Serbian soldiers with a Howitzer during World War I.
Serbian officers with a Howitzer battery as they prepare to fire on the Austrians during the First World War. (circa 1915). (Photo by Hulton Archive/Getty Images)

The 20th century was dominated by war and conflict that continually shifted the balance of power around the globe. This pivotal time period saw the emergence of "total wars" like World War I and World War II in which militaries used any means necessary to win—these wars were so massive they encompassed nearly the entire world. Other wars like the Chinese Civil War remained local but still caused the death of millions.

Motives for these wars ranged from expansion disputes to government upsets, even the intentional murder of an entire people. But they all shared one thing: an extraordinary number of deaths. You will notice that in many cases, soldiers were not the only ones dying.

What Were the Deadliest Wars of the 20th Century?

The three wars of the 1900s with the highest number of civilian and soldier fatalities were World War II, World War I, and the Russian Civil War, respectively.

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.

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). 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.

Other Major Wars and Conflicts of the 20th Century

Many wars, conflicts, revolutions, and genocides shaped the 20th century outside of these top three largest. Take a look at this chronological list of other major wars of the 20th century to see just how much this century was impacted by battle.

1898–1901 Boxer Rebellion
1899–1902
Boer War
1904–1905
Russo-Japanese War
1910–1920
Mexican Revolution
1912–1913
First and Second Balkan Wars
1914–1918 World War I
1915–1918
Armenian Genocide
1917 Russian Revolution
1918–1921
Russian Civil War
1919–1921
Irish War of Independence
1927–1937 Chinese Civil War
1933–1945 Holocaust
1935–1936
Second Italo-Abyssinian War (also known as the Second Italo-Ethiopian War or the Abyssinian War)
1936–1939 Spanish Civil War
1939–1945 World War II
1945–1990
Cold War
1946–1949 Chinese Civil War resumes
1946–1954 First Indochina War (also known as the French Indochina War)
1948 Israel War of Independence (also known as the Arab-Israeli War)
1950–1953 Korean War
1954–1962 French-Algerian War
1955–1972 First Sudanese Civil War
1956 Suez Crisis
1959 Cuban Revolution
1959–1975
 Vietnam War
1967
Six-Day War
1979–1989 Soviet-Afghan War
1980–1988 Iran-Iraq War
1990–1991 Persian Gulf War
1991–1995 Third Balkan War
1994 Rwandan Genocide

View Article Sources
  1. Kesternich, Iris, et al. “The Effects of World War II on Economic and Health Outcomes across Europe.” U.S. National Library of Medicine, 3 Mar. 2014, doi:10.1162/REST_a_00353

  2. Jewell, Nicholas P., et al. “Accounting for Civilian Casualties: From the Past to the Future.” Social Science History, vol. 42, no. 3, pp. 379–410., 11 June 2018, doi:10.1017/ssh.2018.9

  3. Broadberry, Stephen, and Mark Harrison, editors. The Economics of World War I. Cambridge University Press, 2005.

  4. “1918 Pandemic (H1N1 Virus).” Flu, Centers for Disease Control and Prevention, 20 Mar. 2019.

  5. “The Russian Civil War.” Military History Monthly, no. 86, Nov. 2017.

  6. "The Civil War." Facts, National Park Service, 6 May 2015.

  7. “Research Starters: Worldwide Deaths in World War II.” The National WWII Museum | New Orleans.