major part of the increased tension and conflict in the Balkans. In 1912 Bulgaria, Serbia, Greece, and Montenegro defeated the Ottoman Empire in the First Balkan War, but the putative allies continued to quarrel among themselves. Then in 1913, the alliance split, and the Serbs, Greeks, and Romanians defeated Bulgaria in the Second Balkan War. Austria-Hungary became the patron of Bulgaria, which now was Serbia's territorial rival in the region, and Germany remained the Ottoman Empire's protector. Russia tied itself more closely to Serbia than it had previously. The complex system of alliances and Great Power support was extremely unstable; among the Balkan parties harboring resentments over past defeats, the Serbs maintained particular animosity toward the Austro-Hungarian annexation of Bosnia and Herzegovina. In June 1914, a Serbian terrorist assassinated Archduke Franz Ferdinand, heir to the throne of Austria-Hungary, which then held the Serbian government responsible. Austria-Hungary delivered an ultimatum to Serbia. Serbia submitted to the first 2 of 3 cases of the ultimatum; the last one, which was rejected, demanded Serbia allow 100,000 Austrio-Hungarian troops to occupy their country. After Serbian rejection of the third clause of the ultimatum, Austria-Hungary responded forcefully. Russia supported Serbia. Once the Serbian response was rejected, the system of alliances began to operate automatically, with Germany supporting Austria-Hungary and France backing Russia. When Germany invaded France through Belgium as dictated by the Schliffen Plan, the conflict escalated into a world war and they were not prepared. Russia at war, 1914–1916 At the outbreak of the war, Tsar Nicholas yielded to pressure and appointed Grand Duke Nicholas as commander in chief of the Russian armies. The Grand Duke, a cousin of the tsar, was competent but had no part in formulating the strategy or appointing commanders. In the initial phase of the war, Russia's offensives into East Prussia drew enough German troops from the western front to allow the French, Belgians, and British to stop the German advance. One of Russia's two invading armies was almost totally destroyed, however, at the disastrous Battle of Tannenberg—the same site at which Lithuanian, Polish, and Moldovan troops had defeated the German Teutonic Knights in 1410. Meanwhile, the Russians turned back an Austrian offensive and pushed into eastern Galicia, the northeastern region of the Austro-Hungarian Empire. The Russians halted a combined German-Austrian winter counteroffensive into Russian Poland, and in early 1915 they pushed more deeply into Galicia. Then in the