The Union of Transylvania with Romania occurred on December 1, 1918, when the representatives of the Transylvanian Romanian National Assembly declared the unification of Transylvania with the Kingdom of Romania. This event was part of a broader movement for national self-determination following the end of World War I, as various ethnic groups sought to establish their own nations. The union was formalized in the Treaty of Trianon in 1920, which recognized Romania's sovereignty over Transylvania. This decision significantly altered the borders of Hungary, which had previously controlled the region, and it remains a topic of historical and political discussion in both Romania and Hungary today.