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. 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 armies were on the advance, dealing the Russians heavy casualties in Galicia and in Poland, forcing it to retreat. Grand Duke Nicholas was sacked from his position as the commander-in-chief and replaced by the Tsar 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.