In the 20th century, 40 million killed in wars, but 160 million by their own governments.

[pampango]

Twentieth-Century governments and their soldiers have killed perhaps forty million people in war: either soldiers (most of them unlucky enough to have been drafted into the mass armies of the twentieth century) or civilians killed in the course of what could be called military operations. But wars have caused only about a fifth of this century’s violent death toll.

Governments and their police have killed perhaps one hundred and sixty million people in time of peace: class enemies, race enemies, political enemies, economic enemies, imagined enemies. You name them, governments have killed them on a scale that could not previously have been imagined. If the twentieth century has seen the growth of material wealth on a previously-inconceivable scale, it has also seen human slaughter at a previously-unimaginable rate.

Call those political leaders whose followers and supporters have slaughtered more than ten million of their fellow humans “members of the Ten-Million Club.” All pre-twentieth century history may (but may not) have seen two members of the Ten-Million Club: Genghis Khan, ruler of the twelfth century Mongols, launcher of bloody invasions of Central Asia and China, and founder of China's Yuan Dynasty; and Hong Xiuquan, the mid-nineteenth-century Chinese intellectual whose visions convinced him that he was Jesus Christ’s younger brother and who launched the Taiping Rebellion that turned south-central China into a slaughterhouse for decades in the middle of the nineteenth century. Others do not make the list. Napoleon does not make it, and neither does Alexander the Great or Julius Caesar.

By contrast the twentieth century has seen five or six people join the Ten Million Club: Adolf Hitler, Chiang Kaishek, Vladimir Lenin, Joseph Stalin, Mao Zedong, and Tojo Hideki. Hitler, Stalin, and Mao have credentials that may well make them the charter members of the Thirty Million Club as well—and perhaps the Fifty Million Club. A regime whose hands are as bloody as those of the 1965-1998 Suharto regime in Indonesia—with perhaps 450,000 communists, suspected communists, and others in the wrong place at the wrong time dead at its creation in 1965, and perhaps 150,000 inhabitants of East Timor dead since the Indonesian annexation in the mid-1970s—barely makes the twentieth century's top twenty list of civilian-massacring regimes.

http://delong.typepad.com/sdj/2013/04/tyrannies-saturday-twentieth-century-economic-history-weblogging.html

The number for people killed in wars in the 20th century seems low to me. But even if it were doubled, it is amazing that governments have killed their own people in much higher numbers than have died at the hands of foreign armies.