Following defeat in WWI, the Kingdom of Hungary suffered a series of territorial losses, both as retribution for the war and as a result of deep seated ethnic and political rivalries. In philatelic terms, this resulted in a proliferation of overprinted issues. Pre-war Hungary stamps were used as short-term stop gaps while the situation remained fluid.
Included amongst these are: the 1916 issues following Admiral Horthy's recapture of Budapest from the Romanians; followed by his 1919 'National Government' issues from Szeged in the south of the country; the 1918 'People's Republic' issues; the 1920 Occupation of Arad province by the French, prior to it being incorporated into Romania; plus the 1919 Occupation of Banat Bacska by the Serbs, together with the provinces of Debrecen, Temesvar & Baranya.
By far the largest number of overprinted Hungary stamps were used following the annexation of Transylvania by Romania (later ratified by the 1920 Treaty of Trianon), and these provide an interesting area of collection with many varieties of overprint, plus colour and printing errors to intrigue the enthusiast.