Chetniks in World War I were members of auxiliary units used by the Royal Serbian Army for special operations against invading Austro-Hungarian, Bulgarian and German forces.

With the occupation of the country, they evolved into resistance fighters, forming new units and conducting guerrilla warfare, ultimately playing a significant role in the liberation of Serbia and Montenegro.

[2] Throughout the Balkan Wars, Chetnik units played significant roles, acting as Vanguards, disrupting enemy lines, attacking communication facilities, and establishing order in liberated areas.

[5] Each detachment, placed under the command of Serbian officers, consisted of one to three battalions, while predominantly Serbs, Chetnik units also included other South Slavs of various confessions.

[5] The main force of the Serbian army was to be kept inland, leaving regionally based second and third levy units,[a] supported by Chetnik detachments, to defend the borders and conduct reconnaissance.

Upon crossing the Drina River from Bosnia, on the first day of the offensive, on 12 August 1914, the Austro-Hungarian Balkanstreitkräfte faced immediate opposition from Chetnik detachments and border defence troops.

[17] On 15 August, as the Fifth Army moved up the Jadar valley where it mostly encountered Chetnik resistance; the Rudnik Detachment managed to stop the Austro Hungarian 42nd Home Guard Infantry Division near Krupanj.

Despite attempts by the Serbian Supreme Command to prevent premature large-scale guerrilla warfare,[3] when rumors spread that the Allies had reached Skopje and the Bulgarians began conscripting all men between 18 and 45 years of age for military service in January–February 1917, the Serbs spontaneously rose in revolt.

Serbian Chetniks, led by Vojinović, attacked garrisons in the region of Toplica, seizing a number of towns and villages including Prokuplje, Lebane and Kuršumlija.

[26][26] The Bulgarian Supreme Command appointed Macedonia born, IMRO leader Alexander Protogerov, giving him full power to suppress the uprising.

[27] The reprisals did not stop the Chetniks and in April 1917, Pećanac's guerrillas attacked a railway station,[31] and on 15 May, they invaded Bosilegrad before retreating to Kosovo, controlled by the Austro-Hungarians.

In response, the Austro-Hungarian command formed Albanian paramilitary units to hunt down the remaining Serbian rebels,[32] as well counter-units composed of IMRO comitadjis sent from Macedonia.

[33] The following year, Serbian Chetnik units, including the survivors of the Jablanica Detachment,[27] were again instrumental in the liberation of Serbia, as advances were made on the Salonika front.

[3] Chetnik units sustained losses of around 60% of their forces,[6] their use in direct frontal combat often led to devastating casualties, prompting some historians to suggest that they were sacrificed.

[34] In July 1914, prior to the invasion, the Habsburg army determined that Serb Chetniks, or Komitadjis, as the Austro-Hungarians called them, were "outside international law" and were to be "completely wiped out".

[35] On 13 August, the Balkanstreitkräfte's Commander in Chief, General Potiorek ordered all units to seize hostages, carry out reprisal hangings, and engage in arson as part of a punitive strategy, in retaliation for Chetnik raids.