Although the formal elaboration and study of international relations (IR) began only in the 20th century, people have been thinking systematically about world politics for far lon ger. The unsentimental power politics emphasis of Realism in the present era has its antecedents in the writings of Thucydides and Sun Tzu, as well as later thinkers such as Niccolo Machiavelli of the 16th century. Likewise, the idealistic view of human nature and the possibility of human progress propounded by Liberalism is rooted in the writings of such Enlightenment philosophers as Immanuel Kant, John Locke and Jean-Jacques Rousseau — as well as Thomas Jefferson and other founders of American democracy.
The Liberal perspective was dominant in many intellectual circles in the aftermath of World War I (which in that optimistic interlude was called “the War to End All Wars”), when President Woodrow Wilson and other idealists promoted the League of Nations, treaties abolishing war and the like. However, the obvious failure of such efforts to prevent the Second World War helped bring Realism into the prominence it held throughout the Cold War.
Deeper theoretical explorations and the emergence of new empirical challenges, particularly after the end of the Cold War, have brought changes in IR theory. Among the new factors are: multi state political formations (e.g., the European Community), the growing influence of nongovernmental domestic forces in foreign policy decision making, the sudden prominence of non-state international terrorism, the globalization of commerce and communications, the potential of private individuals to influence international behavior (e.g, the Land Mines Treaty) and demands by previously ignored voices of women and minorities seeking to gain a hearing for their perspectives on international behavior. The summary comments below present only a brief (and inevitably simplistic) sampling of a rich and growing intellectual field. Suggested readings at the end of each segment guide the interested student toward major contemporary works dealing with specific theories.