This chapter examines the core assumptions of liberalism regarding world politics. It explores why liberals believe in progress, what explains the ascendancy of liberal ideas in world politics since 1945, and whether liberal solutions to global problems are hard to achieve and difficult to sustain. The chapter also considers central ideas in liberal thinking on international relations, including internationalism, idealism, and institutionalism. It concludes with an assessment of the challenges confronting liberalism. Two case studies are presented: one dealing with imperialism and internationalism in nineteenth-century Britain, and the other with the 1990–1991 Gulf War and its implications for collective security. There is also an Opposing Opinions box that asks whether liberal internationalist governments have a responsibility to protect other people from atrocity crimes.