ON WARS, by Michael Mann If wars are “the least rational of human projects,” why have there been so many of them all over the world, in every era? This is the question that the sociologist Michael Mann poses in the boldly titled “On Wars.” It is an ambitious book, plumbing the roots of war from the early Roman Republic to Vladimir Putin’s Russia, with intermediary chapters on ancient and imperial China, Mongol conquests, feudal Japan, the carnage of European Christendom, clashes in pre-Columbian and Latin America, the two world…