Rummel estimated the total number of people killed by all governments during the 20th century at 212 million, of which 148 million were killed by Communist regimes from 1917 to 1987. To give some perspective on these numbers, Rummel pointed out that all domestic and foreign wars during the 20th century killed in combat around 41 million. His figures for Communist regimes have been criticized for his methodology and for being higher than those given by most other scholars. In his last book, Rummel increased his estimate to over 272 million innocent, non-combatant civilians who were murdered by their own governments during the 20th century. However, Rummel noted that his 272 million death estimate was his lower, more prudent figure, stating that it "could be over 400,000,000." He concluded that democracy is the form of government least likely to kill its citizens and that democracies do not wage war against each other. This is known as the democratic peace theory.