The tactics employed against the Croats were at least to an extent, a reaction to the terror carried out by the Ustashas, but Croats and Bosniaks living in areas intended to be part of Greater Serbia were to be cleansed of non-Serbs regardless, in accordance with Draža Mihailović's directive of 20 December 1941. However the largest Chetnik massacres took place in eastern Bosnia where they preceded any significant Ustasha operations. Chetnik ethnic cleansing targeted Croat civilians throughout areas of Croatia and Bosnia and Herzegovina, in which Croats were massacred and expelled, such as the Krnjeuša massacre and the Gata massacre, among many others. According to the Croatian historian, Vladimir Žerjavić, Chetnik forces killed between 18,000–32,000 Croats during World War II, mostly civilians. Some historians regard Chetnik actions during this period as constituting genocide.