 Complete the following phrase:  War resumed in September 1939 when Germany and the Soviet Union invaded Poland. Some rationalization of the city tram service followed, and the waiting interval between trams was extended from ten to twelve minutes. Destructive bombing of Königsberg began two years later, in August 1941, a couple of months after the German-Soviet wartime alliance had collapsed, with the German invasion of the Soviet Union. During the next four years the tram service was progressively adapted and reduced in response to deepening economic hardship and, especially from 1944, intensification of bomb damage. By 1945 much of Königsberg was rubble, and in 