Croats and Serbs and Kingdom of Serbia merged in 1918 into the Kingdom of Yugoslavia. The creation of the Kingdom of Yugoslavia led to the formation of stronger ties between Serbs in Croatia and Bosnia and Herzegovina with Serbia. Immediately after 1918, the influx of Serbs from Serbia into the territory of Croatia, in particular in the area of Syrmia and Lika-Krbava County, increased. Relative growth in the number of Serb citizens was recorded in Virovitica (35% increase), and Syrmia and Modruš-Rijeka counties, mainly due to the migration of Serbian war veterans who fought on Macedonian front to Slavonia during agrarian reform which was organized by the authorities. Thus, 25 settlements for volunteers were erected, and 8,000 families settled on the land in the areas of agrarian offices in Osijek and Vukovar. Although most of the Serbian parties in Croatia have been co-operating with Croatian Peasant Party in the struggle against Serbian unitarism for years, following the creation of Banovina of Croatia in 1939, part of Serbs showed a lack of willingness to live in a country with Croat majority. There were also requests for joining the Lika and Kordun districts with the Vrbas Banovina which had Serb majority. World War II Following the Invasion of Yugoslavia in April 1941, the Axis powers occupied the entire territory of the Kingdom of Yugoslavia and established a puppet state, the so-called Independent State of Croatia (NDH) on the territory of present-day Bosnia and Herzegovina and Croatia. The Ustaše government saw Serbs, Jews, Romanis and antifascist Croats as a disruptive element and enemies of the Croatian people, and immediately started with their prosecution. One-third of Serbs were to be killed, one-third expelled and one-third forcibly converted to Catholicism, according to a formula devised by Ustaše ministers. Upon the establishment of the NDH, Ustaše officials immediately began making harsh statements against the Serbs. Although some of the prominent Serbs first offered cooperation to the new Ustaše government, Serbs were outlawed. During the first months of NDH's existence, numerous legal decisions were made against the Serbs: they had to leave the public service, had to move to the northern parts of Zagreb, could walk through Zagreb only during the day, had to wear a badge with the letter "P" (stood for "Pravoslavni", meaning Orthodox Christians), the name of their faith was changed from Serbian Orthodox to Greek-Eastern, and usage of the Cyrillic script was prohibited.