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It goes back to the 'Balkan Wars' in 1912/13, and the break up of the Ottoman Empire in Europe. In the Second Balkan War, Bulgaria was at war with Serbia, and lost, so when WWI started, and Germany allied itself with Austria-Hungary against Serbia, Bulgaria joined in on the German side.
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The main reason that Austria-Hungary declared war on Serbia, and thus started WWI, was the assassination of Archduke Franz Ferdinand on June 28, 1914
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Reacting to the Austrian attack on Serbia, Russia begins full mobilization of its troops. Germany demands that it stop. August 1, 1914 - Germany declares war on Russia.
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Great Britain declined an Ottoman request. A secret treaty was concluded between the Ottoman Empire and the German Empire on August 2, 1914. The Ottoman Empire was to enter the war on the side of the Central Powers one day after the German Empire declared war on Russia.
