Chetniks originally formed as a result of the Serbian struggle against Turks. This movement in Herzegovina was aimed against Turks, in northern Macedonia against Turks and Albanians who sided with Turks. The same stood for Balkan Wars, while in WWI, chetniks' actions were targeted against Austria-Hungary.
After the surrender of the Yugoslav royal army in April 1941, Serb soldiers throughout Yugoslavia organized in the Ravna Gora district of western Serbia under Colonel Dragoljub (Draza) Mihailovic. Mihailovic directed his units to arm themselves and await his orders for the final push. He avoided carried out actions which he judged were of low strategic importance. The reason behind his resolve was the fact that he had been a World War I officer. Serbia itself had lost one quarter of its population in that conflict and the Germans had by 1941 introduced punitive measures against guerrilla activity, 100 Serb civilians were to be executed for for every killed soldier of the Wehrmacht and 50 for each wounded. Mihailovic was being assured by the Allies that an invasion of the Balkans was in the plans and it seems to have been in the works up to 1943, the year of the Conference of Yalta after which Stalin and Churchill decided to split Yugoslavia 50-50 according to influence. That very same year, the Allies abandonned the Chetniks in favour of the Partisans.
By the end of the war, the Chetniks were still important in numbers. Some retreated north to surrender to Anglo-American forces; Mihailovic and his few remaining followers tried to fight their way back to the Ravna Gora to continue to collaborate, but he was captured by the Tito's partisans. In March 1946 Mihailovic was brought to Belgrade, where he was tried and executed on charges of treason. The last remaining Chetnik, Vlado Sipcic, braved the wilderness of the Herzegovina-Montenegro border area before being capture by the Communist secret police in 1957.
The Chetniks were also known for rescuing some 500 US airmen who crashed over Serbia while bombing it in 1944-1945.