The 'Battle of Lijevče Field' was fought between Croat and Četnik forces on the Lijevče Field near Banja Luka in what was then the Independent State of Croatia (30 March/8 April 1945).

In December 1944, the Montenegrin Četnik forces of Lieutenant Colonel Pavle Đurisić started to withdraw from German-occupied Montenegro toward north-eastern Bosnia, where there then took place a meeting with Draza Mihailović and other Četnik leaders. Đurisić was critical of Mihailović's leadership, and decided to move to the west to Slovenia in order to seek Allied protection, whereas Mihailović intended to return to Serbia. A pair of Četnik commanders, Zaharije Ostojić and Petar Baćović, together with the ideologue Dragisa Vasić, joined him. Đurisić arrived at an arrangement with the Croat authorities, Montenegrin separatist and Croat ally Sekula Drljević for safe passage through Croatia, and for this Đurisić was denounced by Mihailović as a traitor. Under the agreement, Đurisić's troops were supposed to join Drljević's Montenegrin National Army and recognise Drljević as the Montenegrin leader.

The Croat armed forces and Drljević apparently intended to use the agreement as a trap for Đurisić, who also did not plan to stick to the agreement and kept moving on his own. This led to open conflict with the Croat armed forces. The first clashes occurred on 30 March around the town of Bosanska Gradiska. The entire Četnik 8^th Montenegrin Army, strengthened with several other Četnik corps, had crossed the Vrbas river by 1 April. The Croat armed forces assembled a sizeable grouping, which was better organised and had a significant advantage in heavy weapons, under the command of General Vladimir Metikoš. This grouping attacked the Cetnik forward units and forced them to withdraw towards the Lijevče Field, to the north of Banja Luka. The Croat armed forces then attacked Đurisić's forces on the Lijevče Field and blocked their route to the west. The Četnik attempts to break through but were unsuccessful, and most of the army surrendered during a major Croat attack on 7/8 April. A smaller detachment, led by Đurisić, tried to break through the Croat lines by moving to the south. However, the desertion of many of his troops and of the partisan forces led Đurisić to make any agreement with the Croat armed forces. He was subsequently executed, along with other Četnik officers.

The bulk of the Montenegrin Četnik forces were recruited into Drljević's army and placed under the command of the Croat armed forces.

Following the Italian surrender to the Allies in September 1943, German forces occupied the area of the Italian governorate of Montenegro within 'Achse' (ii). The Germans co-operated with local Montenegrin separatists, local Četniks, and the Sandzak Moslem militia in controlling Montenegro. As these forces proved to be insufficient for the task, the Germans released the imprisoned Četnik commander Đurisić and organised the Montenegrin Volunteer Corps, with the assistance of Milan Nedić's 'Government of National Salvation' , the puppet administration of Serbia, to fight against the Yugoslav partisans.

Late in 1944, with the German withdrawal from the Balkans and the advance of the Soviet forces and the partisans, the situation for the Četniks in Serbia and Montenegro became increasingly difficult. Mihailović, who favoured continued co-operation with the Germans to ensure the supply of weapons and ammunition for his forces, ordered all Četnik forces, including those under Đurisić's command in Montenegro, to head toward Bosnia. While the Četniks assisted the Germans in keeping control of their lines of communication, Mihailović at the same time tried to win back the support of the western Allies. The Germans withdrew from Montenegro early in December 1944, together with Đurisić's troops. The Montenegrin Četniks were at this point reorganised into three divisions, and included the Sandzak Četniks of Vuk Kalaitović. They numbered some 8,700 men and were accompanied by around 3,000 civilians, mostly family members. After a 35-day march, suffering from cold, hunger and disease, Đurisić's army reached Trebava mountain in north-eastern Bosnia in the middle of February 1945. On joining the rest of the Četniks, Đurisić criticised Mihailović's leadership and advocated a retreat to Slovenia, where they would join other Četnik units and await the arrival of western Allied forces, as opposed to Mihailović's insistence on returning to Serbia. He decided to split from Mihailović and move toward the Slovene coast on the Adriatic Sea. Đurisić was joined by Dragisa Vasić, one of the chief Četnik ideologues and Mihailović's political adviser, by the detachments of Petar Baćović, leader of the Herzegovina Četniks, and by Zaharije Ostojić, leader of the east Bosnian Četniks. Just before his departure, on 18 March, Đurisić was joined by part of the Četnik forces from Serbia.

Đurisić made contact with Nedić, head of the puppet government in German-occupied Serbia, and Dimitrije Ljotić, who agreed with his idea of gathering the Četniks in Slovenia. With the help of the German special envoy in Belgrade, Hermann Neubacher, Nedić and Ljotić secured accommodation for Đurisić's troops and refugees in Slovenia. Đurisić had to reach Bihać in western Bosnia on his own, where Ljotić's forces would meet him and assist his further movement.

The area between north-eastern Bosnia and Slovenia lay within the Independent State of Croatia, a German puppet state. This territory was under the control in part of German and Croat forces, and in part of the partisans. The Germans favoured increased collaboration with the Četniks after the capitulation of Italy, despite the protests of the Croat government, which saw in it a danger to the existence of the own state. The Croat government proposed restricting the collaboration to those Četniks whose gave recognition the Croat state and were Croat citizens, and the limiting of their activities to Serb-majority areas, but the Germans refused to accept such complaints. The Croat military forces, namely the Ustase militia and the Croat home guard, were in November 1944 reorganised into the Croat armed forces.

Late in December 1944, Đurisić sent his aide, Dusan Arsović, to Sarajevo to explore the possibility of a joint withdrawal with the Germans. Arsović tried to make contact with Ljubomir Vuksanović, a Montenegrin collaborating with the Germans, but Vuksanović opposed Đurisić and refused to talk to him. Arsović then made contact with representatives of Montenegrin separatist Sekula Drljević, who had spent most of the war in Croatia, and wanted to create an independent Montenegrin state with German and Ustase assistance. The Ustase had pursued close contact with Montenegrin nationalists since the proclamation of the Independent State of Croatia in 1941, when the Montenegrin national committee, headed by Montenegrin writer Savić Marković Stedimlija, was opened in Zagreb, the Croat capital. In the spring of 1944, Drljević relocated from Zemun to Zagreb, where he created an interim government called the Montenegrin State Council. Croatia supported the creation of a Montenegrin state rather than its adherence to Nedić's Serbia in the planned 'New Order of Europe', which was proposed by Neubacher in October 1943.

Talks intensified between Drljević's and Đurisić's representatives after the split with Mihailović. Đurisić saw this as an opportunity to secure safe passage for his army to Slovenia. At a meeting in Doboj, Drljević's negotiators demanded Drljević's recognition as the political leader of the Montenegrins, the recognition of the Montenegrin state council, and the absorption of the Montenegrin Četniks into the Montenegrin national army. Đurisić decided to accept all of Drljević's demands on the condition that his wounded would be accommodated. He also retained operational command of the new army. The agreement was signed on 22 March in Zagreb. However, neither side was sincere in its commitment. Drljević's motive was breaking up the Četnik organisation and the creation of an illusion of strength in German minds, thus giving legitimacy to his idea of Montenegrin independence. He also intended to use the agreement as a trap for Đurisić, who supported the unification of Serbia and Montenegro. The Ustase motive to capture Đurisić was revenge for massacres of Moslems in south-eastern Bosnia and Sandzak committed by Đurisić's Lim-Sandzak Četnik detachment. On the other hand, Đurisić did not want Drljević to have any real control over his army. On 22 March, Drljević sent Đurisić a leaflet for distribution among his soldiers. In this leaflet were details of the agreement, and Drljević referred to himself as the 'supreme commander of the Montenegrin National Army', but Đurisić refused to disseminate the leaflet to his troops.

Drljević arranged the accommodation of wounded and sick with the Croat authorities. Đurisić handed them over in Bosanski Brod, and the wounded were from there transferred to Stara Gradiska. Estimates of their number range from 800 to 2,700. The rest of the army was resting outside Bosanski Brod. On 23 March, Mihailović found out about the deal and immediately informed other Četnik commanders that Đurisić had committed treason and ordered them not to help his forces in their retreat. He stripped Đurisić of his rank, announced that there would be a trial for treason, and said that he would inform the western Allies of Đurisić's siding with the Ustase and Montenegrin separatists. Having heard of Mihailović's reaction, and without consulting with Drljević, Đurisić ordered his forces to start moving to the west.

By the terms of the agreement, Đurisić's army was obliged to cross the Sava river into Slavonia, and from Slavonski Brod continue the march in the direction of Zagreb as the Montenegrin national army. Instead, Đurisić kept moving on a route to the south of the river toward the Motajica mountain and the Lijevče Field, a large plain between the Bosna, Vrbas and Sava rivers. Drljević qualified this action as a violation of the agreement and informed the Croat armed forces' leadership of it. The Croat armed force issued an order to secure the road linking Bosanska Gradiska and Banja Luka, toward which Đurisić's army was heading. The Croat leadership considered that Mihailović was behind Đurisić's action, and that his ultimate goal was to unite with Ljotić's Serbian Volunteer Corps and the Serbian State Guard in Slovenia, and then to attack Zagreb. This move would have allegedly bring them back the support of the western Allies.

North-eastern Bosnia lay within the area of responsibility of the IV Ustase Corps under the command of Josip Metzger. The corps included the Croat 4^th Division, stationed in Dvor, which numbered 7,000 men under the command of Colonel Zdenko Begić, and the Croat 6^th Division in Banja Luka under the command of Metikoš with 4,000 men. These forces were assisted by local militias and the Ustase Defence Corps. It is estimated that a total of between 10,000 and 12,000 troops, under the overall command of Metikoš, were involved in the 'Battle of the Lijevče Field'. Colonel Marko Pavlović, commander of the Ustase Defence units, was also involved in the battle.

On departing Montenegro in January 1945, the Četnik units were reformed into divisions and regiments. The Montenegrin Četniks were organised into the 1^st, 5^th and 8^th Divisions, each of two regiments, and the independent Youth Regiment and Staff Battalion. The Mileseva Corps from Sandzak, led by Vuk Kalaitović, was not re-formed and remained an independent unit. The 1^st Division numbered 2,000 men, the 5^th Division 2,400 men, the 8^th Division 2,200 men, the Staff Battalion 600 men, the Youth Regiment 300 men, Đurisić's Personal Escort, Security and Logistics 800 men, and the Mileseva Corps 400 men. The Drina Corps of Baja Nikić was with Đurisić from 1943, and in March 1945 was strengthened with the Romanija Corps and Četniks from Herzegovina, as well as elements of three corps from Serbia: the Mlava Corps, led by Captain Jagos Zivković, the Kosovo Corps led by Major General Blazo Brajović, and the Rudnik Corps led by Captain Dragomir Topalović 'Gaga'. A part of the Mileseva Corps, around 200 men, split from the main group that returned to Sandzak and remained with Đurisić. The Middle-Bosnian Corps arrived in the later part of the battle.

The main army was referred to as the Četnik 8^th Montenegrin Army, and Četniks from Herzegovina formed the 9^th Division. The Croat armed forces' headquarters estimated that Đurisić's army, on the way to the Lijevče Field, totalled between 10,000 and 12,000 troops, while partisan reports estimated that Đurisić had around 10,000 troops once he reached eastern Bosnia in the middle of February.

On 30 March, the Četnik forces crossed the Vrbas river and took the village of Razboj. From there, the Mileseva Corps attacked the village of Doline on the right bank of the Sava river. The local militia in Doline was forced to retreat, and the bulk of Đurisić's forces continued their march. Đurisić planned to capture the town of Bosanska Gradiska and secure supplies for his army. By 1 April, all of his units had crossed the Vrbas river and reached Razboj. The army then rested for two days as it awaited reinforcements from the Middle-Bosnian Corps.

Units of the Croat 6^th Division and the Ustase Defence Force were sent to stop the advance of Đurisić's leading units. On 2 April, Metikoš's forces attacked the Mileseva and the Drina Corps. The Četniks suffered heavy casualties and many were taken prisoner, including Captain Sima Mijusković, who was a supporter of Drljević's idea of Montenegrin independence. Mijusković gave the Croats information about the number and distribution of the Cetnik forces. A large group of Cetniks, who presented themselves as Montenegrin nationalists and supporters of Drljević, surrendered to the Croats on the night of 2/3 April. At the same time, there was turmoil between Četnik officers in the main group as a result of their leading units' heavy losses. Đurisić abandoned the idea of launching a piecemeal attack on Bosanska Gradiska, and instead directed his forces toward the village of Topola, to the south of the town.

Croat aircraft dropped leaflets urging the Montenegrins to abandon Đurisić and join them and Drljević in the fight against the partisans. Units of the Yugoslav partisan 2^nd Army were currently positioned to the east of the Vrbas river, and Metikoš wanted to finish the battle as soon as possible in order to forestall a possible partisan attack on Banja Luka. He gathered a strong force, with a significant advantage in heavy artillery and tanks, for the task, and Metikoš also ordered the building and strengthening of bunkers on the road linking Bosanska Gradiska and Banja Luka.

On the morning of 4 April, the Croat forces attacked Đurisić's troops on the Lijevče Field. A battalion under Major Antun Vrban was detached toward Kozara mountain to keep watch against possible partisan attacks. After a short battle, the Croats defeated the Gacko Brigade of the Herzegovina Četniks, who were sited in prepared positions, blocking the path to the west. The commander of the Gacko Brigade, Radojica Perisić, was killed in the attack. The Četnik leadership prepared for a quick counterattack. The 5^th Division was sent to capture the village of Aleksandrovac and stop the Croat advance from Banja Luka, the 1^st Division was tasked with capturing the village of Topola to the north, and the rest of the force was to make a breakthrough at the village of Sibića Han. All three villages were situated on the now well-protected road linking Bosanska Gradiska and Banja Luka road. Đurisić's further plan was to secure a path through partisan-held Kozara.

Đurisić tried to negotiate a truce with the Croats and free passage toward Kozara. After the Croats had rejected the request, Đurisić ordered an all-out attack on the following night. Equipped only with small arms and light weapons, the Četniks began a frontal assault on the Croat positions at around 02.00 on 5 April. Initially, the attack achieved some success. Aleksandrovac was captured and held briefly until Croat troops from Banja Luka, reinforced with tanks, recaptured it at dawn. By then, the Četnik attack had been largely repelled by Croat artillery and bunkered defences. Several smaller Četnik detachments managed to pass through the Croat lines and threatened the isolated Vrban’s battalion. At the same time, the battalion came under artillery fire from the partisans' 2^nd Army, so a tank company was sent to assist it. Četnik units also came under partisan bombardment. Đurisić's attempts of entering into negotiations with Metikos were unsuccessful as the latter demanded unconditional surrender. Fighting lasted throughout the night of 5/6 April, during which additional Croat reinforcements arrived.

The Četniks of the Gacko and Nevesinje Brigades, which broke through, reached the slopes of Kozara mountain. As the main Četnik attack failed, these units found themselves isolated between the partisans and the Croats, and soon came under attack from both. They were forced to retreat back to the main group. Of the initial strength of some 800 men, only 220 made it back. Its commander, Milorad Popović, was among the dead. In the meantime, the entire 1^st Regiment of the 1^st Division surrendered and switched allegiance to Drljević.

As a result of the build-up near Banja Luka of the partisan forces, whose 6^th Croat Division was largely involved in the fighting on the Lijevče Field, the Croats decided to launch a final attack on Đurisić's now-demoralised forces. Faced with desertions and an epidemic of typhus, Đurisić abandoned the idea of a full breakthrough, and instead created a detachment of several hundred men under his own command, crossed the Vrbas river once more, and started moving toward Banja Luka with the intention of bypassing it from the south. The rest of Đurisić's forces were supposed to feign acceptance of an agreement with Drljević and try to reach Slovenia.

The attempt of the main army failed, and during the morning of 7 April the Coats concentrated their forces near Razboj and began an artillery bombardment of Četnik positions at about 11.00, followed by a combined armour and infantry assault. The tanks and armoured vehicles broke the Četnik right flank, held by the Drina Corps, and circled to their rear. Unable to hold their positions and surrounded on all sides, the Četniks began to surrender. At the same time, a spoiling attack was being made on the partisans of Koča Popović, to the east of the Vrbas river, to disrupt them during the main battle, which came to an end on 8 April.

Đurisić's detachment of between 500 and 600 men included the entire Mileseva Corps of 200 men. With him were, among others, Vasić, Ostojić and Baćović. When the detachment approached Banja Luka, Kalaitović's Mileseva Corps split from Đurisić and turned for Sandzak. On 10 April, the detachment crossed the Vrbanja river and in Čelinac met with leaders of the Middle Bosnian Četniks, Slavoljub Vranjesević and Lazar Tesanović. The two were ready to join Đurisić, but their troops did not want to leave their villages. Đurisić kept moving down the right bank of the Vrbas river and crossed it at the village of Gornji Seher, to the south of Banja Luka, on 17 April. Đurisić's surviving force was intercepted by the Croats on the road linking Banja Luka and Mrkonjić Grad and given the option of returning to the original agreement the commander had reached with Drljević. The partisans cut off the route to the west and endangered his detachment, so Đurisić accepted the proposal. The detachment was immediately disarmed and sent to Stara Gradiska, where the rest of the surrendered forces were located.

The number of casualties is difficult to determine. Četnik sources mention several hundred men killed, while the Croat General Vjekoslav Luburić after the war mentioned thousands of men killed, which was probably an exaggeration. A German report of 9 April recorded the fact that 5,500 Četniks surrendered in the area of Bosanska Gradiska.

The surrendered Četniks and refugees accompanying them were moved to a barracks in Stara Gradiska. They were allowed to keep their weapons, but were strongly guarded. Two days later, Drljević visited the barracks and gave a speech in which he urged them to be loyal to an independent Montenegro. A selection of Serbs from Montenegrins was then made by a commission set up by Drljević, with Croat assistance. Men not from Montenegro or Herzegovina were segregated and imprisoned by the Croats, these men including those that were recognised as opponents of the Montenegrin separatists. The rest were impressed as three brigades of the Montenegrin national army. Colonel Bosko Agram, a member of the Montenegrin federalist party, was named its commander. The officers admitted to the new army had to pledge allegiance to an independent Montenegro. The new army was then transferred to the area of the II Ustase Corps in Sisak under Luburić, and the V Ustase Corps in Karlovac under General Ivan Herenčić. On 17 April, Drljević outlined his agenda as the fight against a new Yugoslavia, the Četniks of Mihailović, and Soviet influence. Achieving Montenegrin statehood was outlined as the main war objective.

While events after the capture of Đurisić are unclear, sources agree that he and his officers were executed in the area of the Jasenovac concentration camp. Among the total of about 150 who were killed were Ostojić, Baćović and Vasić.

In May 1945, the Montenegrin national army retreated toward Austria together with the Croat armed forces. A large number of its men defied Drljević and Agram once they had crossed the border into Slovenia, leaving only a small number loyal to Drljević. Their attempt to surrender to the British forces at Bleiburg was rejected and the men were instead repatriated to Yugoslavia, where many of them were then killed by the partisans. Drljević managed to evade capture and was interned at a camp in Austria, but was killed in the autumn of 1945 by former members of the Montenegrin Četniks.