Hungarian-Czechoslovak war
Few days before the end of the WWI Austria-Hungary split up and many countries including Czechoslovakia declared independence. However, Kingdom of Hungary refused to recognize Czechoslovakia as souverign state and therefore Hungary did not accept czechoslovak demands on northern part of Hungary (today Slovakia and Carpathian Ruthenia). This escalated into war between Hungary and Czechoslovakia. In this video you can see, how this war changed borders of Czechoslovakia and Hungary.
