-
Austria-hungary
The main reason that Austria-Hungary declared war on Serbia, and thus started WWI, was the assassination of Archduke Franz Ferdinand. -
Germany
Germany entered World War I because countries that were its allies entered the war first. -
Ottoman Empire
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. -
Bulgaria
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.