The Treaty of Trianon is the peace treaty signed in 1920 between the Allies and Hungary.
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. The partition of Hungary brought irredentism, since many people in the lost lands were Hungarian and wanted to continue being part of Hungary.
As a result, about 400 000 people from the lost territories sought refuge in Hungary.
Also, the treaty forced Hungary to pay reparations and to limit the size of its army.