The colonization of Kosovo was a programme begun by the kingdoms of Montenegro and Serbia in the early twentieth century and later implemented by their successor state Yugoslavia at certain periods of time from the interwar era (1918–1941) until 1999. 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 substitute them with Montenegrins and Serbs. Albanians formed the ethnic majority in the region after it became part of Yugoslavia in early 20th century.