Alliances between states, from the Delian League to NATO, are a recurring feature in international relations and they can keep peace or encourage war. States make alliances out of self-interest, fear, or ideology, and the ensuing relationships are rarely easy, especially when they are put to the test. Alliances can make and keep the peace or lead to conflict and in so doing have helped to shape, for better or worse, the modern world. Using examples from the recent past and the present, Margaret MacMillan examines the nature, dynamics, and types of alliances, and suggests reasons why they succeed or fail.