I know it doesn't seem as if it's going to be a brief history if I begin with the Hungarian conquest of the Carpathian Basin in 895-896. But it is necessary because one must mention that before the arrival of the Hungarians the population of the Carpathian basin was mostly Slavic. Archaeological findings attest to this fact, but even in the Hungarian language there are tellings signs: the incredible number of early Slavic borrowings including such an important verb as "to speak" (beszélni). The word "tót," today a somewhat prejorative word for "Slovak," harks back to those days when the Slavs living in the area called themselves "teut" meaning "people." Geographic names endure: people may come and go, but the names of settlements, rivers, mountains stubbornly remain. And many, many place names in today's Hungary are of Slavic origin–for instance, Veszprém and Lake Balaton ("blato" in proto-Slavic meant marshland, mud).
So there is no question that just prior to the arrival of the Hungarians some kind of proto-Slavic population inhabited the area. The Romanians' presence in Transylvania is much more controversial, mostly because the Romanians claim that they are the direct descendants of those Roman legionnaires who were sent by Rome to defend the short-lived Roman province of Dacia occupied by Celts and Germanic tribes around 270. Hungarian scholars claim that the original population of the province couldn't have been Romanized so thoroughly in about 150 years by members of a relatively small Roman garrison. Moreover, it is difficult to imagine how this Romanized population managed to survive centuries and centuries of all sorts of vicissitudes. Hungarian sources first talk of "Vlachs" (later oláhs) in Transylvania only in the thirteenth century. To my mind it is also telling that originally the Romanian name for Transylvania came from the Hungarian "Erdély" meaning "across the mountains." The word "Transylvania" in Romanian appeared only in the late eighteenth century when there was a serious attempt to Latinize the language that is also full of Slavic words and a goodly number of Hungarian borrowings. In any case, whatever the pedigree of the Romanians, the important thing is that eventually the majority of the population of Transylvania became Romanian.
Initially the Hungarians inhabited the Great Plains and Transdanubia and only slowly moved north- and eastward. In those days there was no attempt to Magyarize the population, so the northern areas known in Hungarian as Felvidék (Uplands) remained overwhelmingly Slovak. Moreover, Romanians continued to move into Transylvania and gained a demographic advantage with their very high birth rate. In addition, the Turkish onslaught against Hungary in the sixteenth century and the subsequent 150-year occupation hit the central parts of the country the hardest. The Uplands remained in Habsburg hands, and therefore its development wasn't retarded. Transylvania became a semi-independent principality paying tribute to the Turks but remained free from Turkish occupation. Thus the most uniformly Hungarian-speaking territories suffered the most. A lot of people escaped to non-Turkish territories. Agriculture, industry, and commerce suffered. The incredible loss of population left the land barren. All in all, the Hungarians found themselves a minority in their country.
Fast forward to the end of the nineteenth century and language distribution in Greater Hungary as an indicator of the makeup of the population. I am excluding Croatia-Slavonia since it was not considered an integral part of Hungary and had its own government responsible for home affairs. It could also use Croatian as its official language. Moreover, in 1918-1920 Hungary never contested Croatia's secession from the Lands of St. Stephen. It had the right to do so according to an agreement between the two countries at the end of the thirteenth century.
In 1900 51.4% of the population was Hungarian-speaking. A decade later this percentage increased to 54.5%. But the number of Hungarians might have been exaggerated because the census takers didn't ask about mother tongue (that is, an individual's first language) but what language they spoke best. And because the possibilities for higher education in Slovak or Romanian didn't really exist Slovaks especially assimilated rapidly to Hungarian culture. Another impetus for Slovak assimilation was their mobility. The Uplands were poor and many thousands of Slovaks moved south, especially to Budapest, in search of a better life. And in a big city a generation later the Slovak family's son and daughter spoke and felt Hungarian. This was less the case with the Romanians in Transylvania perhaps because of religion. The Romanians were Orthodox Christians as opposed to the Slovaks who were either Catholic or Lutheran. Or perhaps because Transylvania was more isolated from the rest of Hungary. Whatever the case, Romanians didn't contribute to the growth in "Hungarians" according to the 1910 census. On the contrary, between 1990 and 1910 the Romanian population increased by 5.3%; in 1990 there were 2,798,559 Romanians; by 1910 their numbers had reached 2,948,186.
This was roughly the ethnic composition of the country when Hungary lost the war. Yes, the borders were always drawn in favor of the successor states. Sure, there could have been, especially in the case of the Hungarian-Czechoslovak border, an ethnically more defensible line that would have left about half a million Hungarians within Hungary. This was to my mind a serious mistake committed by the overly demanding Czechs and the not too wise Great Powers. Drawing the Romanian-Hungarian border was much more complicated since the bulk of the Hungarians lived in the middle of Transylvania, surrounded by solidly Romanian territories.
I didn't say anything about the Voivodina (Bánát-Bácska) in the south, today belonging to Serbia, or Western Hungary (today called Burgenland) belonging to Austria. Both are interesting; perhaps I will write about the history of these territories some other time.