Complex sentence: In 1920, after the Treaty of Trianon, Košice (Kassa in Hungarian) became part of Czechoslovakia, Szikszó became the capital of Abaúj-Torna county and held this rank until the unification of the three counties creating Borsod-Abaúj-Zemplén county.
Split into simpler sentences: In 1920, after the Treaty of Trianon, Košice (Kassa in Hungarian) became part of Czechoslovakia, and Szikszó became the capital of Abaúj-Torna county. It held this rank until the unification of the three counties created Borsod-Abaúj-Zemplén county.