The Chetnik Detachments of the Yugoslav Arm, often known as Chetniks, were Serbian irregular forces that during WWII partially fought against the occupation by Germany and allies, but who mainly fought against Communist irregular forces, sometimes even joining Germany and allies against Communists.
There may be various dubious claims related to the Chetniks for reasons such as Allied psychological warfare, notably by Communist Yugoslavia.
More recently, the term “Chetnik” has been used during the break-up of Yugoslavia. Some Serb nationalists, viewing the term positively, used it to describe certain forces that fought for the Bosnian Serbs. Some opponents, viewing the term negatively, used it to describe all pro-Serb armed units.