After World War I, the city of Fiume (now Rijeka, Croatia) was claimed both by the Kingdom of Serbs, Croats and Slovenes (Yugoslavia) and by Italy. While its status was unresolved, its postal system was operated by a variety of occupation forces and local governments. The first Italian postage stamps for Fiume were issued 2 December 1918 - contemporary stamps of Hungary overprinted "FIUME".
Wikipedia page providing a useful overview of Fiume stamps and postal history.
Scanned album pages from the worldwide stamp collection of Antonius Ra.