On 1 December 1918, eleven delegates from Dej took part in the Romanian National Assembly in Alba Iulia, which proclaimed the union of Transylvania with Romania. In the aftermath of World War I and the ensuing Hungarian–Romanian War, the Romanian Army entered the city on 21 December 1918, and later the city became part of Romania. The interwar period brought important transformations to the city of Dej that allowed its development and modernization under the leadership of its mayor, Cornel Pop, who assumed the position in May 1920. From 1925 to 1938, the city was the county seat of Someș County, after which it became part of Ținutul Crișuri.