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1878–1914: Congress of Berlin, Balkan instability and the Bosnia Crisis

Russian Pan-Slavic organizations sent aid to the Balkan rebels and so pressured the tsar's government to declare war on the Ottoman Empire in 1877 in the name of protecting Orthodox Christians. Unable to mediate between the Ottoman Empire and Russia over the control of Serbia, Austria–Hungary declared neutrality when the conflict between the two powers escalated into a war. With help from Romania and Greece, Russia defeated the Ottomans and with the Treaty of San Stefano tried to create a large pro-Russian Bulgaria.

This treaty sparked an international uproar that almost resulted in a general European war. Austria–Hungary and Britain feared that a large Bulgaria would become a Russian satellite that would enable the tsar to dominate the Balkans. British prime minister Benjamin Disraeli moved warships into position against Russia to halt the advance of Russian influence in the eastern Mediterranean so close to Britain's route through the Suez Canal. The Treaty of San Stefano was seen in Austria as much too favourable for Russia and its Orthodox-Slavic goals.

The Congress of Berlin rolled back the Russian victory by partitioning the large Bulgarian state that Russia had carved out of Ottoman territory and denying any part of Bulgaria full independence from the Ottomans. The Congress of Berlin in 1878 let Austria occupy (but not annex) the province of Bosnia and Herzegovina, a predominantly Slavic area. Austria occupied Bosnia and Herzegovina as a way of gaining power in the Balkans. Serbia, Montenegro and Romania became fully independent. Nonetheless, the Balkans remained a site of political unrest with teeming ambition for independence and great power rivalries. At the Congress of Berlin in 1878 Gyula Andrássy (Minister of Foreign Affairs) managed to force Russia to retreat from further demands in the Balkans. As a result, Greater Bulgaria was broken up and Serbian independence was guaranteed. In that year, with Britain's support, Austria–Hungary stationed troops in Bosnia to prevent the Russians from expanding into nearby Serbia. In another measure to keep the Russians out of the Balkans, Austria–Hungary formed an alliance, the Mediterranean Entente, with Britain and Italy in 1887 and concluded mutual defence pacts with Germany in 1879 and Romania in 1883 against a possible Russian attack. Following the Congress of Berlin the European powers attempted to guarantee stability through a complex series of alliances and treaties.

Anxious about Balkan instability and Russian aggression, and to counter French interests in Europe, Austria–Hungary forged a defensive alliance with Germany in October 1879 and in May 1882. In October 1882 Italy joined this partnership in the Triple Alliance largely because of Italy's imperial rivalries with France. Tensions between Russia and Austria–Hungary remained high, so Bismarck replaced the League of the Three Emperors with the Reinsurance Treaty with Russia to keep the Habsburgs from recklessly starting a war over Pan-Slavism. The Sandžak-Raška / Novibazar region was under Austro-Hungarian occupation between 1878 and 1909, when it was returned to the Ottoman Empire, before being ultimately divided between kingdoms of Montenegro and Serbia.

On the heels of the Great Balkan Crisis, Austro-Hungarian forces occupied Bosnia and Herzegovina in August 1878 and the monarchy eventually annexed Bosnia and Herzegovina in October 1908 as a common holding of Cisleithania and Transleithania under the control of the Imperial & Royal finance ministry rather than attaching it to either territorial government. The annexation in 1908 led some in Vienna to contemplate combining Bosnia and Herzegovina with Croatia to form a third Slavic component of the monarchy. The deaths of Franz Joseph's brother, Maximilian (1867), and his only son, Rudolf, made the Emperor's nephew, Franz Ferdinand, heir to the throne. The Archduke was rumoured to have been an advocate for this trialism as a means to limit the power of the Hungarian aristocracy.

A proclamation issued on the occasion of its annexation to the Habsburg monarchy in October 1908 promised these lands constitutional institutions, which should secure to their inhabitants full civil rights and a share in the management of their own affairs by means of a local representative assembly. In performance of this promise a constitution was promulgated in 1910.

The principal players in the Bosnian Crisis of 1908-09 were the foreign ministers of Austria and Russia, Alois Lexa von Aehrenthal and Alexander Izvolsky. Both were motivated by political ambition; the first would emerge successful, and the latter would be broken by the crisis. Along the way, they would drag Europe to the brink of war in 1909. They would also divide Europe into the two armed camps that would go to war in July 1914.

Aehrenthal had started with the assumption that the Slavic minorities could never come together, and the Balkan League would never cause any damage to Austria. He turned down an Ottoman proposal for an alliance that would include Austria, Turkey, and Romania. However, his policies alienated the Bulgarians, who turned instead to Russia and Serbia. Although Austria had no intention to embark on additional expansion to the south, Aehrenthal encouraged speculation to that effect, expecting that it would paralyze the Balkan states. Instead, it incited them to feverish activity to create a defensive block to stop Austria. A series of grave miscalculations at the highest level thus significantly strengthened Austria's enemies.

In 1914, Slavic militants in Bosnia rejected Austria's plan to fully absorb the area; they assassinated the Austrian heir and precipitated World War I.