Empirical research confirms Machiavelli’s assessment. Kroenig presents data about the international system covering 1816 to 2007 indicating that democracies tend to acquire more economic, diplomatic, and military strength than autocracies. They “are also more likely to rank among the ‘major powers’” and “are more likely to become the most powerful state in the system.” The results are all the more striking, Kroenig notes, considering the limited number of democracies: “Democracies’ strangle-hold on global hegemony occurs despite the fact that, throughout this time period, democracies have been rarer than autocracies, making up only about 35 percent of all the observations in the data.”

History provides still more dramatic evidence for Kroenig’s democratic advantage thesis. Case studies are limited because until the second half of the 20th century, democracies were rare. Nevertheless, Kroenig identifies seven seminal great-power rivalries stretching across more than two millennia pitting democracy against autocracy: classic Athens against Persia and then Sparta; the Roman Republic against Carthage and then Macedon; in the Middle Ages and stretching to the dawn of modernity, the Venetian Republic against the Byzantine Empire and then the Duchy of Milan; the 16th- and 17th -century Dutch Republic against the Spanish Empire; the 19th century clash between Great Britain and France; and, in the 20th century, the United Kingdom against Germany and the United States against the Soviet Union.