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.
Romanian armies in the Battle of Stalingrad
In October, Romania joined the Axis and expressed its availability for a military campaign against the Soviet Union, in order to recapture the provinces ceded in June. After a highly successful summer campaign in 1941 as part of Army Group South, the Romanian Armed Forces regained the territory between the Prut and Dniestr rivers. General Antonescu decided to continue to advance alongside the Wehrmacht, disregarding the Romanian High Command's doubts over the possibility of sustaining a mobile warfare campaign deep inside Soviet territory. In October 1941, the Romanian Fourth Army occupied Odessa after a protracted siege which caused more than 80,000 casualties on the Romanian side, severe destruction and many casualties among the civilian population (the Odessa massacre). The spring and summer of 1942 saw the Third and Fourth Romanian Armies in action in the Battle of Crimea and the Battle of the Caucasus. By the fall of 1942, the two armies were poised to join the attack on Stalingrad.