The Eastern Front or Eastern Theater of World War I was a theater of operations that encompassed at its greatest extent the entire frontier between the Russian Empire and Romania on one side and the Austro-Hungarian Empire, Bulgaria, the Ottoman Empire and the German Empire on the other. It stretched from the Baltic Sea in the north to the Black Sea in the south, involved most of Eastern Europe, and stretched deep into Central Europe as well. The term contrasts with "Western Front", which was being fought in Belgium and France.
During 1910, Russian General Yuri Danilov developed "Plan 19" under which four armies would invade East Prussia. This plan was criticised as Austria-Hungary could be a greater threat than the German Empire. So instead of four armies invading East Prussia, the Russians planned to send two armies to East Prussia, and two armies to defend against Austro-Hungarian forces invading from Galicia. In the opening months of the war, the Imperial Russian Army attempted an invasion of eastern Prussia in the northwestern theater, only to be beaten back by the Germans after some initial success.