During World War I the town was the site of the Battle of Gumbinnen, a major battle on the Eastern Front. The battle took place nearby in the opening days of the war in August 1914, and Gumbinnen was subsequently occupied by the Russian Imperial Army for several months. After the war, a power plant, the Ostpreußenwerk, was built in Gumbinnen and powered much of East Prussia. At the beginning of the Nazi era, Gumbinnen was designated a military sub-region of the Königsberg military area. Near the end of World War II, in 1944, the first of Gumbinnen's 24,000 residents began to flee from the advancing Red Army, and a Soviet air attack on October 16, 1944, caused heavy damage to the inner town area. On October 22, 1944, the town was taken by Soviet forces, who engaged in numerous vengeful atrocities against the civilian population before the Wehrmacht retook the town two days later. Although the German forces retook Gumbinnen and managed in late October to stabilize the battle line east of the town, it was quickly re-conquered by the Red Army during the great Soviet East Prussia offensive on 21 January 1945. During the Evacuation of East Prussia, the surviving German residents fled or were forcibly expelled.