The First Battle of the Masurian Lakes was a German offensive in the Eastern Front during the early stages of World War I. It pushed the Russian First Army back across its entire front, eventually ejecting it from Germany in disarray. Further progress was hampered by the arrival of the Russian Tenth Army on the Germans' left flank. Although not as devastating as the Battle of Tannenberg that took place a week earlier,[citation needed] the battle nevertheless upset Russian plans into the spring of 1915.