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.
Ion Antonescu
As the Soviet Union recovered from the initial shock and slowed down the Axis offensive at the Battle of Moscow (October 1941 – January 1942), Romania was asked by its allies to contribute a larger number of troops. A decisive factor in Antonescu's compliance with the request appears to have been a special visit to Bucharest by Wehrmacht commander Wilhelm Keitel, who introduced the "Conducător" to Hitler's plan for attacking the Caucasus ("see Battle of the Caucasus"). The Romanian force engaged in the war reportedly exceeded German demands. It came to around 500,000 troops and thirty actively involved divisions. As a sign of his satisfaction, Hitler presented his Romanian counterpart with a luxury car. On December 7, 1941, after reflecting on the possibility for Romania, Hungary and Finland to change their stance, the British government responded to repeated Soviet requests and declared war on all three countries. Following Japan's attack on Pearl Harbor and in compliance with its Axis commitment, Romania declared war on the United States within five days. These developments contrasted with Antonescu's own statement of December 7: "I am an ally of the [German] Reich against [the Soviet Union], I am neutral in the conflict between Great Britain and Germany. I am for America against the Japanese."