The Bulgars fought alongside the Russian Army in the 1877–78 Russo-Turkish war and were rewarded with the creation of an autonomous Bulgarian principality in the Treaty of San Stefano. However, the other Great Powers intervened and drastically pruned the territory of the new state, leaving many ethnic Bulgars outside its borders. Bulgaria became aggressively expansionist, earning the sobriquet of the ‘Balkan Prussia’. It found its first outlet in war with Serbia in 1885, then annexed Eastern Rumelia. In 1912 Bulgaria joined Serbia, Montenegro and Greece to declare war on the Ottomans. The war liberated Thrace and Macedonia but there were arguments over the division of the spoils. In 1913, Bulgaria declared war on Greece and Serbia, followed by Romania. Outgunned, Bulgaria was forced to sue for peace, ceding the bulk of Macedonia to Serbia and Greece, and Southern Dobruja to Romania. Bulgaria joined the Central Powers in September 1915.
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