Eastern Front, major theatre of combat during World War I that included operations on the main Russian front as well as campaigns in Romania. The principal belligerents were Russia and Romania (of the Allied and Associated Powers) versus the Central Powers countries of Germany, Austria-Hungary, and Bulgaria. The Eastern Front, which stretched from the Baltic Sea in the north to the Black Sea in the south, was more than twice as long as the Western Front. The greater distances and greater differences between the equipment and quality of the armies involved ensured a fluidity of combat which was lacking in the west.