Whatever harm the 1939 Molotov-Ribbentrop Pact did to Europe and the world, it rescued the institution of the Russian Orthodox Church. Hitler's deal with Stalin allowed the Soviets to occupy eastern Poland, and 1,200 Orthodox parishes were incorporated into the Soviet Union as a result. The communists soon started closing churches and arresting priests and lay Christians in the newly acquired lands, but they also understood that the Russian Orthodox Church could be an instrument of assimilation and of Soviet control. When Hitler launched his invasion, German forces advanced with great speed along a thousand-mile front stretching from the Baltic Sea to the Black Sea. Life for the surviving Orthodox people in areas under continuing Soviet rule remained extremely hard, particularly in the blockaded city of Leningrad. During the siege, the priest and deacon at the Transfiguration Church lived in the church's cellar.