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
In summer 1940 a series of territorial disputes were diplomatically resolved unfavorably to Romania, resulting in the loss of most of the territory gained in the wake of World War I. This caused the popularity of Romania's government to plummet, further reinforcing the fascist and military factions, who eventually staged a coup that turned the country into a dictatorship under "Mareșal" Ion Antonescu. The new regime firmly set the country on a course towards the Axis camp, officially joining the Axis powers on 23 November 1940. As a member of the Axis, Romania joined the invasion of the Soviet Union on 22 June 1941, providing equipment and oil to Nazi Germany as well as committing more troops to the Eastern Front than all the other allies of Germany combined. Romanian forces played a large role during fighting in Ukraine, Bessarabia, Stalingrad and elsewhere. Romanian troops were responsible for the persecution and massacre of up to 260,000 Jews in Romanian-controlled territories, though most Jews living within Romania survived the harsh conditions. According to historian and author Mark Axworthy, the second Axis army in Europe, arguably, belonged to Romania, though, this is disputed since many would agree that this position goes to the Italian army.