This book began by focusing on the causes of war and the potential for peace, the complexities of world politics and the emergence of globalizing forces throughout the 20th century expanded the scope of International Relations to include the study of human rights, migration, environmental cooperation, economic development, ethnic conflict, nationalism, terrorism, and international crime. Today, International Relations scholars have developed sophisticated theories and models in order to study an ever-expanding set of issues and concerns. Constituting one of the main subfields in political science, International Relations continues to demonstrate how political power defines this growing set of issues and concerns. This book explores historical development of international relations theory throughout the 20th century and early 21st Century.