Common sense alone suggests that embarking upon a military adventure is simpler for a king or dictator than it is for a head of government answerable to a parliament. But proponents of this idea argue that a war between democracies has never occurred. One arrives at this comforting conclusion chiefly by setting up the rules to exclude all the non-conforming cases. The most egregious omission is civil wars, which account for a high percentage of the world's violent conflicts--159 of 575 wars between 1816 and 1980, by one count. The spectacle of the American Civil War, in which two popularly elected governments engaged in four years of the most brutal slaughter, refutes the bald notion that citizens will not vote for politicians who send them off to be killed.