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The study of International Relations in political science gained strength during the Cold War. Many political scientists feared that the Soviet Union and the United States would launch themselves into a global nuclear war. It was hoped that by studying how countries interact with each other, it would be easier to predict what the reactions of the United States or Soviet Union would be in any given situation, making it easier to avert war.
The study of International Relations has been, until very recently, the study of how states interact with other states. However, the New World Order of the post-Cold War state system has required a redefinition of actors.
Liberal International Theory vs. International Realism Theory
Liberal theory is often marked by an analysis of an ideal, or utopian, outcome. Realist theory, however, focuses on the ways and means that states interact. An excellent discussion on the differences of the natures of these two theories can be found in Edwards H. Carr's "The Twenty Years' Crisis, 1919-1939" (he uses the term 'utopianism' for 'liberalism') or in Hans Morgenthau's "Politics Among Nations."
Realists often credit Thucydides as the first realist thinker for his desire to leave an account of war as a "possession for all time," his writing style in which he was only concerned with facts from reliable sources, and for his concentration on power in "The History of the Peloponnesian War." The Melian Dialogue is often given extra attention due to the unique use of dialogue, leading many to believe that Thucydides is trying to highlight an important event. From this comes the quote "The strong do what they have the power to do and the weak accept what they must," expressing the realist theme of might-equals-right. However, for an interesting look at the worldview that shaped Thucydides' writing, see RG Collingwood's "The Idea of History."
Realism has traditionally focused on power, and in the Westphalian state system, the state is seen as having the most power. Because it is concerned with power, realism has tended to focus more on high politics than low politics. However, with most wars since the early 1990s being fought within states instead of between states, this is currently being redefined within realism.
Other formative thinkers in the realist tradition include:
- St Augustine of Hippo (The City of God)
- Niccolò Machiavelli (The Prince)
- Thomas Hobbes (Leviathan)
- Carl Schmitt (Concept of the Political)
- Hans Morgenthau (Politics Among Nations)
- Henry Kissinger
- Hedley Bull (The Anarchical Society)
- Kenneth Waltz
- John Mearsheimer (The Tragedy of Great Power Politics)
Liberalism (also called "Idealism" pejoratively) gained popularity following the end of World War I and the triumph of Wilsonian liberalism. Liberalism in international relations theory is generally associated with internationalism, institutionalism, and socialism. Essentially, they believe human nature is good, war can be avoided, and there is a moral universalism. They do not trace a long history of their theory, like realists do. Key themes include international law, sovereignty, international organisations, and global governance.
Formative thinkers in the liberal tradition include:
- Francisco de Vitoria & the School of Salamanca (De Indis debates) 
- Immanuel Kant (Perpetual Peace) w:Perpetual_Peace
- Vladimir Lenin (Imperialism: The Highest Stage of Capitalism)
- David Held (Democracy and the Global Order)
- Prologue- An Introduction to International Relations & Strategic Studies.
- Evolution of the Modern State System- Evolution of the Modern State System
- World Politics in the Twentieth Century- World Politics in the Twentieth Century
- Core Concepts of International Relations- Core Concepts of International Relations
- Theories of I.R.- Approaches to International Relations: Theories of I.R.
- Core Concepts of Strategic Studies- Core Concepts of Strategic Studies
- The Use of Force- The Use of Force
- The Causes of War- The Causes of War
- The Making of Foreign Policy- The Making of Foreign Policy
- International Organisations- International Organisations
- Regionalisation- Regionalisation
- World Government Theories- World Governance Theories
- The United Nations- An International Peace-Keeping Organisation?
- The European Union- An "Area of Peace, Security, and Justice".
- Environment and World Politics- Global Warming & Other Natural Threats.
- War and Peace in a Globalised World- Globalisation & its Consequences on World Conflicts.
- International Political Economy- International Political Economy