Michael Green, a history professor, explains that it's difficult to determine the exact number of deaths in World War I, as estimates vary and some include deaths from related events like the 1918 flu pandemic and the Armenian genocide.
According to a 2011 report, over 16.5 million people, including 9.7 million military personnel and 6.8 million civilians, died during World War I.
The victors of the war suffered more military deaths than the losers, with the Allied side losing 5.4 million military personnel, including 1.8 million from Russia.
The Central Powers lost approximately four million military members, considerably less than the over 5.4 million lost by the Allied forces, including Britain, France, Russia, Italy, Serbia, and the U.S., among others.
The actual death toll of World War I may have been higher than estimated due to the difficulty of accurately recording the number of casualties, and advanced warfare technology such as trench warfare, submarines, airplanes, poison gas, flamethrowers, and machine guns contributed to the high death toll.
Obsolete tactics led to massacres as infantry charged across open fields at artillery and machine guns, notes French historian Antoine Prost.
French Army lost 310,000 soldiers in the first four months of World War I, leading Charles de Gaulle to later write about the futility of bravery against firepower.
Artillery shells were the biggest lethal threat on the new battlefield of World War I, causing 60% of wounded French soldiers to be injured, according to historian Antoine Prost.
The inability of either side to quickly win the war exacerbated the death toll, leading to brutal, protracted struggles such as the Battle of the Somme, which claimed 20,000 British soldiers on the first day of fighting alone.