Just to back up Denes a bit.
After the WW I, Hungary defeated and without a leader, a new government rose from the ruins. Its leader was Mihaly Karolyi, who was a pacifist and indeed had no intention to fight either the Petite nor the normal Entente, he wanted peace even at the costs of territory. But, after the so called Vix memorandum, which demanded Hungary to give the major part of its eastern area to Romania, his government rapidly collapsed and a new one, a communist, emerged. Lead by a jew, Bela Kun, this new state eventualy began to take back the places captured by the czechs, romanians and serbs, who used the pacifism of Karolyi to do whatever they wanted with the borders of the state which has already lost the most of its area and population. The new army, the Red one, fought the czechs and stopped their advance in the territory which is now southern Slovakia, but which has more than 75% Hungarians of the population. But, when Romania began its advance at the east, the communists ran away, hiding in the chaos of the Soviet Union. So, literally nothing stood between the Romanian army and Budapest, nothing but one man, admiral Horthy, who later lost his reputation for co-operating with the nazi regime. The admiral formed a new army from the former Austrian-Hungarian army's veterans in south Hungary, and eventually re-took the most of the lost area, including the already pillaged capital, Budapest. The Romanian troops retreated back to their country, but they still held Transylvania, a complicated area which is very mixed between the two nations, at some place you might find a 100% population of Hungarians but at other places just Romanians. This is the true story of the Romanian army in Budapest.
Hope that none of you will take offense of this post, since I wrote the very best and the most peacefull I could of this serious part in the history of our States.