The Chetniks (Serbo-Croatian Cyrillic: Четници, Serbo-Croatian Latin: Četnici, pronounced [tʃɛ̂tniːtsi]; Slovene: Četniki), formally the Chetnik Detachments of the Yugoslav Army, and also the Yugoslav Army in the Homeland and the Ravna Gora Movement, was a Yugoslav royalist and Serbian nationalist movement and guerrilla force in Axis-occupied Yugoslavia. Although it was not a homogeneous movement, it was led by Draža Mihailović. While it was anti-Axis in its long-term goals and engaged in marginal resistance activities for limited periods, it also engaged in tactical or selective collaboration with the occupying forces for almost all of the war. The Chetnik movement adopted a policy of collaboration with regard to the Axis, and engaged in cooperation to one degree or another by establishing modus vivendi or operating as "legalised" auxiliary forces under Axis control. Over a period of time, and in different parts of the country, the movement was progressively drawn into collaboration agreements: first with the puppet Government of National Salvation in the German-occupied territory of Serbia, then with the Italians in occupied Dalmatia and Montenegro, with some of the Ustaše forces in northern Bosnia, and, after the Italian capitulation in September 1943, with the Germans directly.
The Chetniks were active in the uprising in the German-occupied territory of Serbia from July to December 1941. As a result of the Battle of Loznica at the end of August, Mihailović's Chetniks were the first to liberate a European city from Axis control. Following the initial success of the uprising, the German occupiers enacted Adolf Hitler's formula for suppressing anti-Nazi resistance in Eastern Europe, a ratio of 100 hostages executed for every German soldier killed and 50 hostages executed for every soldier wounded. In October 1941, German soldiers and Serbian collaborators perpetrated two massacres against civilians in Kraljevo and Kragujevac, with a combined death toll reaching over 4,500 civilians, most of whom were Serbs. This convinced Mihailović that killing German troops would only result in further unnecessary deaths of tens of thousands of Serbs. As a result, he decided to scale back Chetnik guerrilla attacks and wait for an Allied landing in the Balkans. While Chetnik collaboration reached "extensive and systematic" proportions, the Chetniks themselves referred to their policy of collaboration as "using the enemy". The political scientist Sabrina Ramet has observed, "Both the Chetniks' political program and the extent of their collaboration have been amply, even voluminously, documented; it is more than a bit disappointing, thus, that people can still be found who believe that the Chetniks were doing anything besides attempting to realize a vision of an ethnically homogeneous Greater Serbian state, which they intended to advance, in the short run, by a policy of collaboration with the Axis forces".
The Chetniks were partners in the pattern of terror and counter-terror that developed in Yugoslavia during World War II. They used terror tactics against Croats in areas where Serbs and Croats were intermixed, against the Muslim population in Bosnia, Herzegovina and Sandžak, and against the Communist-led Yugoslav Partisans and their supporters in all areas. These tactics included the killing of civilians, burning of villages, assassinations and destruction of property and exacerbating existing ethnic tensions between Croats and Serbs. The terror tactics against the Croats were, to at least an extent, a reaction to the terror carried out by the Ustaše, however the largest Chetnik massacres took place in eastern Bosnia where they preceded any significant Ustashe operations. Croats and Bosniaks living in areas intended to be part of Greater Serbia were to be cleansed of non-Serbs regardless, in accordance with Mihailović's directive of 20 December 1941. The terror against the communist Partisans and their supporters was ideologically driven. Several historians regard Chetnik actions during this period as constituting genocide. Estimates of the number of deaths caused by the Chetniks in Croatia and Bosnia and Herzegovina range from 50,000 to 68,000, while more than 5,000 victims are registered in the region of Sandžak. About 300 villages and small towns were destroyed, along with a large number of mosques and Catholic churches.