 Complete the following phrase:  Military action began on the Eastern Front on 17 August 1914 with the Battle of Stallupönen during the Russian invasion of East Prussia. As German army was concentrated on the Western Front, Russians occupied about two-thirds of Prussia and stood just away from Königsberg. It was the only German Empire territory, apart from Alsace-Lorraine on the Western Front, that saw direct military actions during the war. Russians were defeated in the Battle of Tannenberg and retreated, but counterattacked in October 1914. This time, they captured about one-fifth of Prussia. The front stabilized until February 1915 when Russians were driven out during the 