Treaty of Trianon
Hungary lost World War I and so the treaty gave about two thirds of its land and one third of its population to neighboring countries, like Romania, Czechoslovakia and Yugoslavia. Under the principle of self-determination the majority of the people in the lost territories were something other than Hungarian. The partition brought irredentism, since many people in the lost lands were Hungarian and wanted to continue being part of Hungary.  Hundreds of thousands of people from the lost territories sought refuge in Hungary.