Upvote:1
The grandpowers (France, Britain, Italy) that decided about the borders of Hungary in 1919-1920 did not take into account the ethnic division lines at all. About 1/3 of Hungarians found themselves outside of Hungary, and many regions that were predominantly Hungarian were attached to other countries (some other examples are Székelyföld which was given to Romania or the Csallóköz which was given to Czechoslovakia - today Slovakia). They not even allowed for the local people to vote if they really wanted to join the state to which their territory was given (one notable exception is Sopron).
The reasons were complex.
Often railways or important commercial roads were passing through these areas, and they did not want Hungary to control these lines.
Czechoslovakia and Romania wanted to get as much land from Hungary as possible, even many areas where hardly any Slovaks or Romanians were living. The grandpowers favoured them instead of trying to follow the ethnic borders.
In the case of Transcarpathia, the majority of the local population (not just that 20-30% who were ethnically Hungarian and were living in the strip next to Hungary) was kind of loyal to Hungary in 1920. But Benes wanted to get that land and attach it to Czechoslovakia to have a connection to Romania. This was important for him because he wanted to organise a coalition against Hungary (called Little Entente), and he wanted a direct border between Czechoslovakia and Romania (BTW, he also wanted a corridor between Czechoslovakia and Yugoslavia to completely surround Hungary. This area is now called Burgenland, and it is part of Austria, as the idea was rejected by the grandpowers). The main point is that he wanted Czechoslovakia to control the Cop (Csap)-Berehove (Beregszász)- Vinogradov (Nagyszőlős) railway line.