Between 1939 and 1941, in an attempt to resolve the Croat-Serb political and social antagonism in first Yugoslavia, the Kingdom established an autonomous Banovina of Croatia incorporating (amongst other territories) much of the former Military Frontier as well as parts of Bosnia and Herzegovina. In 1941, the Axis powers invaded Yugoslavia and in the aftermath the Independent State of Croatia (which included the whole of today's Bosnia and Herzegovina and parts of Serbia (Eastern Syrmia) as well) was declared. The Germans installed the Ustaše (who had allegedly plotted the assassination of the Serbian King Alexander I of Yugoslavia in 1934) as rulers of the new country; the Ustaše authorities promptly pursued a genocidal policy of persecution of Serbs, Jews and Croats (from opposition groups), leading to hundreds of thousands being killed. During this period, individual Croats coalesced around the ruling authorities or around the communist anti-fascist Partisans. Serbs from around the Knin area tended to join the Chetniks, whilst Serbs from the Banovina and Slavonia regions tended to join the Partisans. Various Chetnik groups also committed atrocities against Croats across many areas of Lika and parts of northern Dalmatia. Many historians regard Chetnik crimes against Muslims and Croats during this period as also constituting genocide.