The Serbian Football Championship season of 1919-1920 was the first held after the end of the First World War. With the football championships still not organised at national level in the newly formed Kingdom of Serbs, Croats and Slovenes (later renamed into Yugoslavia in 1929), the clubs from Serbia joined together to play in this championship organised by the Serbian Football Federation (Srpski loptački savez).
References
1919–20 Serbian Football Championship Wikipedia(Text) CC BY-SA