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. The first case study looks at the rise and fall of what it terms ‘the indispensable nation’, in other words the old liberal world order, lead by the United States. The other case study examines the African concept of sovereignty: sovereignty as responsibility.