The HOS assembled a large force that was better organized and had a significant advantage in heavy weapons, under the command of General Vladimir Metikoš.

As these forces did not prove to be sufficient, the Germans released the imprisoned Chetnik Commander Pavle Đurišić and organized the Montenegrin Volunteer Corps, with the assistance of Nedić's government, to fight against the Yugoslav Partisans.

[1] In late 1944, with the German withdrawal from the Balkans and the advance of the Red Army and the Partisans, the situation for the Chetniks in Serbia and Montenegro became increasingly difficult.

Mihailović, who favoured continued cooperation with the Germans to obtain weapons and ammunition, ordered all Chetnik forces, including those under Đurišić's command in Montenegro, to head towards Bosnia.

While the Chetniks assisted the Germans in keeping control of the communication lines, Mihailović at the same time tried to win back the support of the Western Allies.

After a 35-day march, suffering from cold, hunger and diseases, Đurišić's army reached the Trebava mountain in northeast Bosnia in mid-February 1945.

[7] Đurišić made contact with Milan Nedić, head of the puppet government in German-occupied Serbia, and Dimitrije Ljotić, who agreed with his idea of gathering the Chetniks in Slovenia.

With the help of the German Special Envoy in Belgrade, Hermann Neubacher, Nedić and Ljotić secured accommodation for Đurišić's troops and refugees in Slovenia.

[9] The Germans favoured increased collaboration with the Chetniks after the capitulation of Italy, despite protests from the NDH government which saw in it a danger to the existence of the state.

The NDH supported the creation of a Montenegrin state rather than it joining Nedić's Serbia in the planned New Order of Europe,[12] which was proposed by Neubacher in October 1943.

Drljević's motive was breaking up the Chetnik organization and creating an illusion of strength to the Germans, thus giving legitimacy to his idea of Montenegrin independence.

In it were details of the agreement, and Drljević referred to himself as the "supreme commander of the Montenegrin National Army", but Đurišić refused to spread the leaflet to his troops.

On 23 March, Mihailović found out about the deal and immediately informed other Chetnik commanders that Đurišić committed treason and ordered them not to help him in the retreat.

The Chetniks suffered heavy casualties and many were taken prisoner, including Captain Sima Mijušković, who was a supporter of Drljević's idea of Montenegrin independence.

A large group of Chetniks, who presented themselves as Montenegrin nationalists and supporters od Drljević, surrendered to the HOS on the night of 2/3 April.

Đurišić gave up the planned attack on Bosanska Gradiška, and instead directed his forces towards the village of Topola, south of the town.

[26] The Croatian Air Force dropped leaflets urging the Montenegrins to abandon Đurišić and join them and Drljević in the fight against the Partisans.

A battalion under Major Antun Vrban was set aside towards the Kozara mountain to keep watch against possible Partisan attacks.

After a short battle, the HOS defeated the Gacko Brigade of the Herzegovina Chetniks and encamped in occupied positions, blocking the path to the west.

[22] Equipped with small arms and light weapons, the Chetniks began a frontal assault on HOS positions at around 2am on 5 April.

Aleksandrovac was captured and held briefly until HOS troops from Banja Luka, reinforced with tanks, recaptured it at dawn.

[34] Due to the Partisan troop build-up near Banja Luka, whose 6th Croatian Division was largely involved in the fighting on the Lijevče field, the HOS decided to launch a final attack on the demoralized Đurišić's forces.

[35] He formed a detachment of several hundred troops under his command, crossed the Vrbas again, and started moving towards Banja Luka, intending to bypass it from the south.

[36] The attempt of the main army failed and in the morning of 7 April, the HOS amassed its forces near Razboj and began an artillery bombardment of Chetnik positions at around 11 AM, followed by a combined tank-infantry assault.

At the same time, a spoiling attack was ongoing on the Partisans of Koča Popović, east of Vrbas, to disrupt them while the main battle took place, which ended on 8 April.

On 10 April, the detachment crossed the Vrbanja River and in Čelinac met with leaders of the Middle-Bosnian Chetniks, Slavoljub Vranješević and Lazar Tešanović.

[37] Đurišić kept moving down the right bank of the Vrbas and crossed it at the village of Gornji Šeher, south of Banja Luka, on 17 April.

Soldiers that were not from Montenegro or Herzegovina were set aside and imprisoned by the NDH authorities,[35] including those that were recognized as enemies of Montenegrin separatists.

On 9th of April in a report, partisans claim to have killed 50, drowned 45, 50 wounded and 240 captured Chetniks, which included Đurišić's wife and child and wives of other officers.

Exact fate of prisoners of war is unknown, however considering that local partisan commanders were thinking about conscripting them, most of them were probably spared.