Explanation: Alliances, which were characteristic of the nineteenth and early twentieth centuries, constrained the allied countries to support one another and were one of the major causes of World War I. An attack on Serbia, for example, was considered an attack on Russia and its allies, France and Great Britain. Imperialism is an expansive policy of conquest and control (A). Nationalism refers to national identity (B). Militarism (D) refers to the buildup of armies and materiel.