War has been far too common in human history and thus is the central problem of international relations. Many political scientists and foreign policymakers view war as the continuation of politics: When diplomacy fails, some states decide to use force. Others see war as the result of a breakdown of the modern international system because so many of the rules of international institutions were designed to reduce conflict among states.
Political scientists have long debated the causes of war. These scholars have come up with the following list:
Example: There has been extensive research on whether democracies are less likely to start wars than other regimes. Overall, it appears that democracies are less likely to fight other democracies, a phenomenon scholars refer to as the democratic peace. Democracies are, however, just as likely as other types of regimes to fight nondemocracies.
Example: During the early modern era, nearly every European country experienced numerous wars of religion as the Catholics sought to destroy the Protestants. The wars of religion culminated in the Thirty Years’ War, which stretched from Spain and France to the eastern stretches of Germany during the seventeenth century. It was a brutal and horrific war, and the Catholics’ failure to win the war marked the end of the major religious wars in Europe.