History may not be over yet, but the United Nations and other international organizations are doing their best to end it. If all viable alternatives to liberal democracy have gone the way of the divine right of kings, it is in part because of pressure from international society. Most States now want to participate in the world economy, and are thus complying with international rules requiring that they liberalize their domestic economies. Relatedly, as other contributors to this volume show, international law is moving from its traditional neutrality on States' domestic institutions to a decided preference for liberal democracy. One grounding for this new international–societal pressure is the proposition that liberalism brings peace – more precisely, that liberal States, generally defined as States that limit governmental power via civil rights and competitive elections, do not fight wars against one another. Such States generally engage in war with normal frequency, but their enemies are virtually never fellow liberal States. If the “liberal peace” proposition is true, then international actors charged with making and keeping peace, such as the United Nations, can simplify their tasks by spreading and consolidating liberal government.

The liberal peace proposition is widely accepted as a law-like generalization among political scientists who study international relations. Whether it is actually liberalism that causes the peace is more controversial. Lacking has been a satisfactory account of the causal mechanism linking liberal democracy and peace.