During the First World War, most of today's North Macedonia was part of the Bulgarian occupied zone of Serbia after the country was invaded by the Central Powers in the fall of 1915. The region was known as the “Military Inspection Area of Macedonia” and was administered by a Bulgarian military commander. A policy of Bulgarization of the region and its population was immediately initiated, during the period the IMRO arose from a clandestine organization to serve as gendarmerie, taking control of the whole police structure, enforcing the Bulgarization of the region. According to Robert Gerwarth, the Bulgarian denationalization policy, including its paramilitary aspect, was almost identical in its intent and execution to the Serbian policy that preceded it.