Like with most parts of the Hapsburg empire, Hungary dissolved the personal union with Austria and declared independence after the comprehensive defeat of both Germany and Austria-Hungary.
Hungarian rule included many regions that were minority-Hungarian (e.g. modern Slovakia, Transylvania, Croatia, etc.) Unwilling to give up all of these territories, but unwilling to fight, the ruling government of Count Mihály Ádám György Miklós Károlyi de Nagykároly abolished the entire Hungarian army immediately after independence in an idealistic pacifist-minded mood.
Results of independence and the sudden arrival of trade barriers between e.g. Germany and Hungary led to economic collapse, and increasing Entente territorial demands on Hungary (even if they were areas that were populated mostly by minorities) were vastly unpopular. The result was the attempt of Karolyi to appoint a social democratic government. Instead, the Hungarian Social Democrats chose to merge their party with the (still-imprisoned and illegal) Communists, whose power base was rapidly expanding due to the social breakdown. The Communists were also the only group with an organized fighting force (thanks to Karolyi's pre-emptive disbanding of the army), and promised (rather optimistically) that aid would come from the Soviet Red Army to help restore Hungarian borders.
The end result was the Hungarian Soviet Republic, which attempted to fight Czechoslovakia (for Slovakia) and Romania (for Transylvania) simultaneously, without imposing conscription.) Results were predictable; despite Hungarian successes on the Czechoslovak front, a Romanian breakthrough led to Romanian troops marching through Budapest and Hungarian Communists fleeing the country (eventually for the USSR where they promptly died in the Great Purge).
