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.
Second Romanian Campaign of World War I
The First Romanian Campaign ended in victory for the Central Powers, forcing Romania to sign the Treaty of Bucharest and drop out of the war in May 1918. This treaty was deeply resented by the Romanians, who re-declared war on the Central Powers on 10 November 1918.