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
On 22 June 1941, German armies with Romanian support attacked the Soviet Union. German and Romanian units conquered Bessarabia, Odessa, and Sevastopol, then marched eastward across the Russian steppes toward Stalingrad. Romania welcomed the war because they were allies with Germany. In a morbid competition with Hungary to curry Hitler's favor and hoping to regain northern Transylvania, Romania mustered more combat troops for the Nazi war effort than all of Germany's other allies combined. Hitler rewarded Romania's loyalty by returning Bessarabia and northern Bukovina and by allowing Romania to annex Soviet lands immediately east of the Dniester, including Odessa. Romanian jingoes in Odessa even distributed a geography showing that the Dacians had inhabited most of southern Russia. After recovering Bessarabia and Bukovina (Operation München), Romanian units fought side by side with the Germans onward to Odessa, Sevastopol, Stalingrad and the Caucasus. The total number of troops involved on the Eastern Front with the Romanian Third Army and the Romanian Fourth Army was second only to that of Nazi Germany itself. The Romanian Army had a total of 686,258 men under arms in the summer of 1941 and a total of 1,224,691 men in the summer of 1944. The number of Romanian troops sent to fight in the Soviet Union exceeded that of all of Germany's other allies combined. A "Country Study" by the U.S. Federal Research Division of the Library of Congress attributes this to a "morbid competition with Hungary to curry Hitler's favor... [in hope of]... regaining northern Transylvania."