Before Trianon, Hungary was part of the Austro-Hungarian Monarchy, which stretched over a substantial part of central Europe, including most of present-day Austria, Slovakia, the Czech Republic, Poland, Slovenia, Croatia, Serbia, Romania, and Ukraine as well as Hungary. As a result, it was not strictly regulated where people belonging to any ethnicity should live. Hungarians were indeed concentrated in the Carpathian basin, but there were many settlements in the neighboring regions too.
When the new borders were drawn, victors disregarded not only the historical, cultural, geographic, economic and strategic, but also the ethnic aspects of the region. While territories mostly populated by the various ethnicities were more or less assigned to corresponding successor states, huge chunks of territories obviously populated by ethnic Hungarians were not assigned to Hungary. As a result, many ethnic Hungarians found themselves citizens of “foreign” countries. They faced a choice of leaving their ancestral home and coming to Hungary, or staying in an environment that was often hostile against them.