A world war is generally defined as a major international conflict that involves most or all of the world’s leading powers. The term is most often applied to the two world wars that took place in the first half of the 20th century. Those wars, also known as the Great War or the First World War, were devastating for their participants—losing a generation of young men—and for the countries they left behind. They spelled disaster for Germany, the United States and other Allies as well as for the Ottoman Empire, Austria-Hungary and Russia. By the end of 1918, four of those initial belligerents had been defeated; a fifth—Britain—had lost its empire; and the last, Austria-Hungary—was in turmoil, with Chancellor Friedrich Ebert asking for an armistice to avert civil war and revolution.

Ultimately, the causes of world war were numerous and complicated. Imperialism—the desire of powerful nations to expand their territory and influence—was a significant factor, as were nationalist movements that compelled people to decide their allegiance on the basis of ethnic or cultural identity rather than shared interests. The conflict was also sparked by a series of diplomatic incidents, and exacerbated by secret diplomacy—those “secret sympathies” that fueled tension between European countries.

Yet the decisions that led to the outbreak of the crisis in August 1914 were influenced by much longer-term forces as well, including competing long and short-term foreign policy goals, political pressures at home, and a system of opposing alliances that had evolved over decades. Meticulously researched and masterfully written, The Sleepwalkers tells the story of how these and other forces converged to bring Europe into a world war in just five weeks.