Hungary–Romania relations
During World War II, Hungary and Romania became allies and participated in the war against the Soviet Union. But after the coup on 23 August 1944 Romania switched sides and fought against Hungary. Consequently, Soviet and Romanian troops invaded Hungary, occupied Northern Transylvania by October 1944 and re-established the Romanian administration in the region in March 1945. The 1947 Treaty of Paris reaffirmed the borders between Romania and Hungary as originally defined in Treaty of Trianon 27 years earlier, thus confirming the return of Northern Transylvania to Romania.
Romania in World War II
Romanian soldiers and gendarmes also worked with the "Einsatzkommandos", German killing squads, tasked with massacring Jews and Roma in conquered territories, the local Ukrainian militia, and the SS squads of local Ukrainian Germans (Sonderkommando Russland and Selbstschutz). Romanian troops were in large part responsible for the Odessa massacre, in which from October 18, 1941, until mid-March 1942, Romanian soldiers in Odessa, aided by gendarmes and police, killed up to 25,000 Jews and deported more than 35,000.