Soviet policy was particularly harsh toward the Ukrainian Greek-Catholic Church. Ukrainian Greek-Catholics came under Soviet rule in 1939, when western Ukraine was incorporated into the Soviet Union as part of the Nazi-Soviet Nonaggression Pact. Although the Ukrainian Greek Catholic Church was permitted to function, it was almost immediately subjected to intense harassment. Retreating before the German army in 1941, Soviet authorities arrested large numbers of Ukrainian Greek Catholic priests, who were either killed or deported to Siberia. After the Red Army reoccupied western Ukraine in 1944, the Soviet regime liquidated the Ukrainian Greek-Catholic Church by arresting its metropolitan, all of its bishops, hundreds of clergy, and the more active church members, killing some and sending the rest to labor camps. At the same time, Soviet authorities forced the remaining clergy to abrogate the union with Rome and subordinate themselves to the Russian Orthodox Church.