Cassels traces the part played by ideology in international relations over the past two centuries. Starting with the French Revolution's injection of ideology into interstate politics, he finishes by addressing present-day pre-occupations with the legacy of nationalist discontent left by the collapse of communism and the resurgence of religious fundamentalism in world politics.

Cassels includes discussion of Marxism-Leninism, Fascism and Nazism but, eschewing exclusive focus on totalitarian dogma, he also shows how the interplay of the less rigid belief systems of conservatism, liberalism and nationalism influence international affairs.

The focus and emphasis given to ideology in an historical survey of such broad scope make this book unusual, and even controversial. Social scientific and philosophical discussions of ideology make only glancing reference to foreign policy. Historians have generally touched on ideology only within the context of the case study, while the realist theorists of international relations play down its influence.