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.
Romanian Land Forces
On July 6, 1916, Romania declared war on Germany and Austria-Hungary, following the initial success of the Brusilov Offensive (a major Russian offensive against the armies of the Central Powers on the Eastern Front). The Romanian armies entered Transylvania (then part of the Austro-Hungarian Empire), together with Russian forces. However, German forces under the command of General Erich von Falkenhayn stalled the attack in November, 1916, and drove back the Romanians. At the same time, Austrian and Turkish troops invaded southern Romania, forcing the country into a two-front war. The Central Powers drove deep into Romania and conquered the south of the country (Wallachia, including Bucharest) by the end of 1916. The Romanian forces, led by Marshal Constantin Prezan, retreated into the north-east part of Romania (Moldavia). In the summer of 1917 however, Prezan, aided by the future Marshal, General Ion Antonescu, successfully defended the remaining unoccupied territories against German and Austro-Hungarian forces led by Field Marshal August von Mackensen. General Alexandru Averescu led the Second Army in the victories of the Battle of Mărăşti (July 22 to August 1, 1917) and the Battle of Mărăşeşti (August 6 to September 8, 1917). As a result of the Russian Revolution, Romania was left isolated and unable to continue the war, and was forced to sign the Treaty of Bucharest with the Central Powers. Later on, in 1919, Germany agreed, in the Treaty of Versailles Article 259, to renounce all the benefits provided to it by the Treaty of Bucharest in 1918. After the successful offensive on the Thessaloniki front, which put Bulgaria out of the war, Romania re-entered the war on November 10, 1918, a day before its end in the West.