3 © Springer Nature Switzerland AG 2020 M. O. Hosli, J. Selleslaghs (eds.), The Changing Global Order, United Nations University Series on Regionalism 17, https://doi.org/10.1007/978-3-030-21603-0_1 Chapter 1 Traditional Theories of International Relations Denise de Buck and Madeleine O. Hosli 1.1 Introduction The study of international relations in its early origins aimed to answer questions related to the conditions of peace and warfare. Why and when do nations go to war? What are the underlying motives of warfare? And when are peaceful relations most likely? These questions were particularly prominent in the twentieth century after having experienced two world wars with devastating impacts globally. Although such questions remain central to the discipline today, the causes of peace and warfare are not the only subjects addressed. International relations scholars are equally interested in understanding why international organizations emerged and what role they play on the global stage. They investigate the impact institutions have on existing power structures and whether they constrain the behavior of the most powerful players. Other questions focus on how and when states cooperate and the factors that contrib- ute to hostile or peaceful relations. Regardless of the topic investigated, a crucial tool in understanding global problems, order and relations are theoretical frameworks. International relations theories generate assumptions to explain and predict state behavior and interests. The theories attempt to rationalize why states act the way they do in particular situations. This chapter introduces you to the theories of inter- national relations and illustrates in what way mainstream approaches interpret how global affairs change, what the role of states is in the international system, whether international institutions matter and whether international cooperation between states is both possible and sustainable. Although the chapter content may be a bit D. de Buck Faculty of Governance and Global Affairs, Leiden University, The Hague, The Netherlands e-mail: denise_debuck@hotmail.nl M. O. Hosli (*) Institute of Security and Global Affairs, Leiden University, The Hague, The Netherlands e-mail: m.o.hosli@fgga.leidenuniv.nl