Border conflicts between Poland and Czechoslovakia began in 1918 between the Second Polish Republic and First Czechoslovak Republic, both freshly created states.The conflicts centered on the disputed areas of Cieszyn Silesia, Orava Territory and Spiš.After World War II they broadened to include areas around the cities of Kłodzko and Racibórz, which until 1945 had belonged to Germany.
https://en.wikipedia.org/wiki/Polish%E2%80%93Czechoslovak_border_conflicts