Early approaches to international relations can be found in the works of the Greeks and Romans. Plato and Aristotle, who wrote on the concept of war and the defense of the city-state. Partially as a result of the decline of the Greek city-states, the idealist concept of cosmopolitanism and world citizenship took hold. Roman scholars later developed the law of nations, which consisted of a body of legal principles and practices common to those societies associated with Rome. French writers, particularly those during the Enlightenment Era, focused on the roles of diplomacy, arbitration, and adjudication in the achievement of perpetual peace, and tended to prefer to achieve policies goals through trade and commerce rather than war (Dougherty and Pfaltzgraff, Jr. 2001).
In the 1500s, Jean Bodin (1992) wrote about the principle of sovereignty, which held that a monarch was supreme internally, but equal to other rulers externally. English political philosophers, including Hobbes, Spinoza, and Locke, agreed with the French writers on the concept of sovereignty but not on the prospects for international government. The period of European history from the end of the Thirty Years War in 1648 to the beginning of World War I in 1914 was known as the Golden Age of Diplomacy, and scholarly writings from this time focused on the balance of power, alliances, and international law in a state system characterized by numerous wars (Dougherty and Pfalztgraff, Jr. 2001).
The Inter-War Period