Governments not only interact with the people they rule but also with other governments—to trade, to share ideas, to work together to solve global problems, and to resolve disputes. Political scientists have been analyzing international relations— relations between states—for centuries, but never more so than during the twentieth century, as scholars tried to explain the reasons for and explore the aftermath of World Wars I and II and the Cold War that followed.
Although numerous international agreements and institutions exist to facilitate smooth relations among the nearly 200 countries in the world, international politics can still be extremely violent. Even though people have fought one another for millennia, political scientists still do not know exactly what causes people and states to go to war, start revolutions, or commit acts of terrorism. Identifying both immediate and long-term causes and consequences of political violence, as well as thinking about the impact of this violence on the international system, has become an important part of political science.