Russo-Ukrainian war - Transcarpathia.
Magyars arise. A weak Kiev is an opportunity to free 150,000 ethnic Hungarians who were on the wrong side when the border was redrawn in 1945. The Ukraine could lose the war easily, in which case what guarantee is there for Magyars outside the boundaries of Hungary? Transcarpathia must be readmitted to the Hungarian family - which it was part of for centuries.