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Turkey
When Turkey joined the war they were already on the central powers side because they had an agreement with Ottoman. -
Austria-Hungary
Austria-Hungary joined the war because Archduke Franz Ferdinand was assassinated. Therefore, they gave an ultimatum for Serbia. -
germany
Germany was a big part of why most of the countries joined the war in the first place. the Germans declared war on Russia and France then had Japan and the U.S declared war on them later.