Why was World War II the deadliest war in history?

World War II was the deadliest war in history because it was fought everywhere, by almost everyone, against everyone, civilians included. Nearly all the world's countries took part between 1939 and 1945, many in total war on an unprecedented scale, and somewhere between 60 and 75 million people died. That is about 3% of everyone alive in 1940, and most of them were not soldiers.

That last fact is the key. Military deaths from all causes were around 21 to 25 million. The rest were civilians, killed by massacres, bombing, starvation, disease and genocide. The Holocaust alone murdered about six million European Jews. Famine and disease linked to the war killed another 19 to 28 million people, on top of those killed directly.

The dying was also deeply uneven. More than half of all deaths were in China and the Soviet Union: Russian government figures put Soviet losses at 26.6 million, and China counts 20 million dead. Leningrad endured the most lethal siege in history.

Even the totals are uncertain. The authors of the Oxford Companion to World War II warn that casualty statistics are notoriously unreliable. That is why some people count names rather than numbers: by early 2019, Yad Vashem's database held the names of 4.8 million Jewish victims of the Holocaust, each one recorded individually.