The Battle of Lodz was a battle fought on the Eastern Front of World War I from 11 November to 6 December 1914, when the Imperial German Army launched a major offensive against the Imperial Russian Army in Congress Poland and captured the Polish city of Lodz after heavy fighting. Aiming to prevent a Russian invasion of Germany amid the Battle of the Vistula River, Erich Ludendorff sent the German 9th Army to threaten the Russian northern flank. On 11 November 1914, the German general August von Mackensen attacked the Russians at Lodz, forcing the Russians to halt their advance on Silesia to avoid encirclement, and enabling the Russian Fifth Army to launch a flanking attack on Mackensen's force shortly after it had smashed through the Russian flank. The Russians entrenched themselves in front of Lodz, forcing Ludendorff to obtain reinforcements from the Western Front in order to launch repeated frontal assaults on Lodz. On 29 November, Grand Duke Nicholas ordered a strategic withdrawal to Warsaw, leaving Lodz in German hands after heavy losses on both sides. The Germans entered the industrial city on 6 December 1914, and the onset of winter in Poland brought about a lull in the major fighting.