In 1908, Austria-Hungary announced its annexation of Bosnia and Herzegovina, provinces in the Balkans. Bosnia and Herzegovina had been nominally under the sovereignty of the Ottoman Empire but administered by Austria-Hungary since the Congress of Berlin in 1878. The announcement upset the fragile balance of power in the Balkans and enraged Serbia and pan-Slavic nationalists throughout Europe. The weakened Russia was forced to submit to its humiliation, but its foreign office still viewed Austria-Hungary's actions as overly aggressive and threatening. Russia's response was to encourage pro-Russian and anti-Austrian sentiment in Serbia and other Balkan provinces, provoking Austrian fears of Slavic expansionism in the region.