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
After the tide of war turned against the Axis, Romania was bombed by the Allies from 1943 onwards and invaded by advancing Soviet armies in 1944. With popular support for Romania's participation in the war faltering and German-Romanian fronts collapsing under the Soviet onslaught, King Michael of Romania led a "coup d'état" that deposed the Antonescu regime and put Romania on the side of the Allies for the remainder of the war. Despite this late association with the winning side, Greater Romania was largely dismantled, losing territory to Bulgaria and the Soviet Union, but regaining Northern Transylvania from Hungary.