A kingdom in name only: after Austria-Hungary's 1918 collapse and the 1919 Hungarian Soviet Republic / White Terror chaos, Hungary restored the monarchy as a constitutional form but left the throne vacant, ruled instead by Regent Miklós Horthy from 1920. It functioned as a conservative, revisionist authoritarian state focused on reversing the Treaty of Trianon's territorial losses, drifting into alliance with Nazi Germany through the 1930s–40s.
German forces occupied Hungary on 19 March 1944; Horthy was deposed on 16 October 1944 and the Arrow Cross Party's Ferenc Szálasi installed as 'Nemzetvezető' (Nation Leader) heading a puppet 'Government of National Unity.' Wikipedia's own treatment (and contemporary Hungarian usage) holds that the Arrow Cross regime never abolished the monarchy as a form of government and the country continued to be called the Kingdom of Hungary through the war's end; Budapest fell to Soviet/Romanian forces in February 1945 and the Szálasi government dissolved by May 1945. The monarchy was only formally, legally abolished on 1 February 1946, when the Second Hungarian Republic was proclaimed.
The atlas holds 7 border frames for Kingdom of Hungary (1920–1946) — the shape you scrub through changes in 1920 · 1934 · 1938 · 1939 · 1940 · 1941 · 1945.
Cambridge's standard one-volume history of Hungary, covering the Horthy era and the war years.