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 the aftermath of World War I, Romania, which fought with the Entente against the Central Powers, had greatly expanded its territory, incorporating the regions of Transylvania, Bessarabia and Bukovina, largely as a result of the vacuum created by the collapse of the Austro-Hungarian and Russian empires. This led to the achievement of the long-standing nationalist goal of creating a Greater Romania, a national state that would incorporate all ethnic Romanians. However, the newly gained territories also included significant Hungarian, German, Bulgarian, Ukrainian and Russian minorities, which put Romania at odds with several of her neighbors. This occasionally led to violent conflict, as exemplified by the Hungarian–Romanian War and the Tatarbunary uprising. To contain Hungarian irredentism, Romania, Yugoslavia and Czechoslovakia established the Little Entente in 1921. That same year Romania and Poland concluded a defensive alliance against the emergent Soviet Union, and in 1934 the Balkan Entente was formed with Yugoslavia, Greece and Turkey, which were suspicious of Bulgaria.