I would love to continue the Romanian-Hungarian strand especially after Dumneazu's very interesting comments, but first I would like to talk about the fate of lost Hungarian territories in the south and the west. I already mentioned that Croatia-Slavonia's relation to Hungary was different from the other lost territories. While they were an integral part of the country, Croatia-Slavonia was a separate country that was attached to Hungary proper through the person of the king. This personal union occurred at the end of the eleventh century when the last Croatian king died without issue. His wife was the sister of St. László, king of Hungary. After the death of the king different noble groups vied for power, and one of them supporting the queen invited St. László to occupy the country and take the Croatian throne. This was a similar arrangement to the later personal union between Austria and Hungary after the death of King Louis II in 1526 at the Battle of Mohács. His wife was the sister of Ferdinand, the Austrian archduke, brother of Emperor Charles V of the Holy Roman Empire.
Similar to the Compromise of 1867 between Austria and Hungary, a year later there was a compromise between Croatia and Hungary by which the Croats received wide local autonomy in internal affairs, judiciary, and education. But just as most Hungarians were dissatisfied with their compromise with Austria, the Croatians wanted more than autonomy. They dreamt of a southern Slav state, the brainchild of Croatian intellectuals (the Illyrian movement, named after the Roman province of Illyria) in the middle of the nineteenth century. After the lost war, Hungary acknowledged Croatia-Slavonia's special status within Greater Hungary and did not dispute its secession. So Croatia realized the Illyrian dream. Unfortunately for them it became more of a nightmare. By the end of the 1920s Croatian leaders were complaining to Hungarian diplomats about their situation vis-à-vis the Serbs. And we know today that the Croats were only too eager to break away from Yugoslavia.
To continue the "be careful what you wish for" theme, Hungarian nationalists (the Party of Independence) for a long time had as their goal Hungarian independence from Austria. Except they forgot one very important thing: an independent Hungary and the country's territorial integrity were mutually exclusive concepts. While Budapest was rejoicing over its independence at the end of the war the non-Hungarians within Greater Hungary were desirous of their own independence; they wanted to sever connections with the Magyars of Hungary. The lost war, the Hungarian declaration of independence, the non-Magyars' desire to secede, the greedy neighbors all contributed to the collapse.
But let's go back to the southern parts of the country, first the territory between Croatia and Romania north of Belgrade that the Serbs call Voivodina (Vajdaság/Bánát-Bácska). After the Turks were expelled from these territories the area was pretty much devastated and Leopold II, king of Hungary, encouraged Serbs still under Turkish rule to settle in Hungary (1690-91). About 200,000 followed their religious leaders and settled in the area. Germans and eventually Romanians also started to move here from southern Transylvania. Eventually the area was a veritable ethnic checkerboard. One village Serb, the next Hungarian, its neighbor German or Romanian. In fact, a serious dispute occurred between two allies, Romania and the Yugoslavia, over the territory. They were ready to fight each other for certain parts of today's Voivodina. To make a long story short, the lion's share of this area was given to Yugoslavia although the population was very mixed. According to the 1910 census the population broke down as follows: 450,000 Serbs and other southern Slav people, 442,000 Hungarians, 312,000 Germans, 73,000 Romanians. Today the number of Hungarians is around 300,000.
Hungary also lost to Croatia the so-called Baranya triangle. Baranya is a Hungarian county in southern Hungary, bordering on the Drava river. South of the Drava belonged to Croatia which, as I mentioned, was not in dispute. But the Serbs occupied territories north of the Drava as well, including the capital of Baranya, Pécs, a city that was overwhelmingly Hungarian. In fact, Yugoslavia refused to withdraw from Pécs until 1921. Although Yugoslavia demanded a great deal more territory, eventually they had to settle for only the Baranya triangle, an area at the confluence of the Danube and the Drava.
And then came the big surprise to the Hungarians. The Austrians suddenly realized that if the self-determination of nations were to be taken seriously Nyugat-Magyarország (Western Hungary) should belong to Austria because, after all, it was almost completely German. I think that was a perfectly reasonable demand just as the German parts of Bohemia-Moravia should have been given to Austria on ethnic grounds, but surely that was out of the question. However, the annexation of territory from another vanquished country was much easier. The Austrians, as opposed to the Czechs, Romanians, and Serbs, did not and could not occupy the territories they demanded but patiently waited for the decision in Paris. This time the Hungarians were not too eager to withdraw their troops from the territory. Eventually the Hungarian government managed to convince the Allied and Associated Powers to allow a plebiscite in Sopron, the largest town in the region, and in some of the surrounding villages. The plebiscite went in Hungary's favor. I might add that it was through this area, today's Burgenland, that Edvard Beneš planned a "corridor" that would have connected Czechoslovakia and Yugoslavia. His claim rested on the few thousand Croats living there. Luckily this idea was too much for the peacemakers.
So that's a brief description of how and why the dismemberment of Greater Hungary occurred. The major goal of Hungarian foreign policy between the two world wars was to regain these territories. Although officially the Hungarian government never admitted that they would be satisfied with border adjustments, in reality they would have been quite happy with partial restitution. The German brokered arrangement between Slovakia and Hungary was a relatively fair border, and I think that if the Hungarians hadn't put all their eggs in one basket perhaps it would have stood the test of time. Even some arrangement with Romania could have been worked out. The Soviet Union promised diplomatic backing if Hungary didn't join the Germans in their attack of the Soviet Union. But Hungary was a loyal German ally, and it paid the price.