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 Russia and Romania on one side and Austria-Hungary, Bulgaria, the Ottoman Empire, and Germany 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 Germany after some initial success. At the same time, in the south, they successfully invaded Galicia, defeating the Austro-Hungarian forces there. In Russian Poland, the Germans failed to take Warsaw. But by 1915, the German and Austro-Hungarian forces were on the advance, dealing the Russians heavy casualties in Galicia and in Poland, forcing them to retreat. Grand Duke Nicholas was sacked from his position as the commander-in-chief and replaced by Tsar Nicholas himself. Several offensives against the Germans in 1916 failed, including Lake Naroch Offensive and the Baranovichi Offensive. However, General Aleksei Brusilov oversaw a highly successful operation against Austria-Hungary that became known as the Brusilov offensive, which saw the Russian Army make large gains. Being the largest and most lethal offensive of WW1, the effects of the Brusilov offensive were far reaching. It helped to relieve the German pressure on Battle of Verdun, while also helping to relieve the Austro-Hungarian pressure on the Italians. As a result, the Austro-Hungarian Armed forces were fatally weakened, and finally Romania decided to enter the war on the side of the Allied Forces. However, the Russian human and material losses also greatly contributed to the Russian revolutions. The Kingdom of Romania entered the war in August 1916. The Allied Powers promised the region of Transylvania (which was part of Austria-Hungary) in return for Romanian support. The Romanian