Over the course of the twentieth century, Kosovo experienced four major colonisation campaigns that aimed at altering the ethnic population balance in the region, to decrease the Albanian population and replace them with Serbs and Montenegrins. The colonisation programme began in the early twentieth century between the kingdoms of Serbia and Montenegro during the Balkan Wars and was later implemented by their successor state Yugoslavia at certain periods from the interwar era until 1999. Albanians formed the ethnic majority in the region when it became part of Yugoslavia in the early twentieth century.