Security, Development, industrialization, Crises in the century. International relations, The roots of World War I, 1871–1914, The cold War in Europe, Chinese Civil War, Korea War, Cold War in Middle East. 20th-century international relations, history of the relations between states, especially the great powers, from approximately 1900 to 2000. The history of the 20th century was shaped by the changing relations of the world’s great powers. The first half of the century, the age of the World Wars and the start of the Cold War, was dominated by the rivalries of those powers. The second half saw the replacement, largely through the agency of those wars, of the European state system by a world system with many centres of both power and discord. This article provides a single integrated narrative of the changing context of world politics, from the outbreak of World War I to the 1990s. Because domestic affairs figure heavily in the analysis of each state’s foreign policies, the reader should consult the histories of the individual countries for more detail. Forty-three years of peace among the great powers of Europe came to an end in 1914, when an act of political terrorism provoked two great alliance systems into mortal combat. The South Slav campaign against Austrian rule in Bosnia, culminating in the assassination of the Habsburg heir apparent at Sarajevo, was the spark. This local crisis rapidly engulfed all the powers of Europe through the mechanisms of the Triple Alliance and the Triple Entente, diplomatic arrangements meant precisely to enhance the security of their members and to deter potential aggressors. The long-term causes of the war can therefore be traced to the forces that impelled the formation of those alliances, increased tensions among the great powers, and made at least some European leaders desperate enough to seek their objectives even at the risk of a general war. These forces included militarism and mass mobilization, instability in domestic and international politics occasioned by rapid industrial growth, global imperialism, popular nationalism, and the rise of a social Darwinist worldview. But the question of why World War I broke out should be considered together with the questions of why peace ended and why in 1914 rather than before or after.