'West-Bosnien' was a German, Croat and Hungarian operation against the partisan forces of Josip Broz Tito in the Kozara mountain area between Banja Luka and Prijedor of Axis-occupied Yugoslavia (10 June/17 July 1942).

Known to the Yugoslavs as the Battle of Kozara, 'West-Bosnien' took place in the middle of 1942 on and around the mountain of Kozara in north-western Bosnia, where the 2^nd 'Krajiski' Partisan Detachment was concentrated. In the spring of 1942 the partisan forces in central and western Bosnia had liberated Bosanski Petrovac, Drvar, Glamoč and Prijedor, and on 20 May the 2^nd 'Krajiski' Partisan Detachment had been established, on the following day obtaining a limited number of armoured fighting vehicles and aircraft with which to hold the newly liberated territory stretching from the Sava river to the south across the Kozara and Grmeč mountains.

The German and Croat forces realised that the city of Banja Luka and the iron mine in Ljubija were threatened, and therefore planned 'West-Bosnien' to destroy the partisan forces with forces that fluctuated with time between 35,000 and 40,000 men (15,000 Germans and between 20,000 and 25,000 Croats). The Germans contributed some 15,000 men, the Croats about 22,000 home army and Usta’e soldiers, and the Hungarians their Danube Flotilla (four river monitors, four gunboats and two armoured boats). The German forces comprised Generalmajor Heinrich Borowski’s 704^th Division, elements of Generalleutnant Friedrich Stahl’s 714^th Division, elements of Generalleutnant Johann Fortner’s 718^th Division, the 521^st Nahrichtenregiment, elements of the 202^nd Panzerabteilung, the 924^th Landesschützenbataillon, the 8^th Landesschützenbaubataillon, and the 23^rd Panzerzug armoured train; the Croat forces comprised Pukovnik Artur Gustović's 1^st Mountain Division (with the 1^st and 2^nd Mountain Brigades), 3^rd and 4^th Mountain Brigades, Banja Luka Brigade, 1, 2 and 3/Petrinja Brigade, elements of the 1^st, 2^nd, 5^th, 10^th, 11^th and 12^th Regiments, elements of the 1^st Domobran Assault Battalion, the 4^th, 5^th, 6^th, 7^th, 8^th, 12^th, 13^th and 18^th Ustase Battalions, the Sanski Most, Kotoriste, Brozani Majdan and Banja Luka Volunteer Battalions, the Suhača Volunteer Company, the sPrinz Eugens Volksdeutsch Battalion, the sLudwig von Badens Volksdeutsch Battalion, the 1^st, 7^th, 10^th and 11^th Recruit Battalions, elements of the 1^st, 3^rd, 4^th and 8^th Artillery Groups, two pioneer companies, one railway engineer company, the 4^th Motor Transport Battalion, the 2^nd Gendarmerie Battalion, about 2,000 Četnik auxiliaries, the Air Group Cenić (single fighter, bomber and mixed squadrons), and the 1^st Air Force Field Company.

The partisans totalled about 3,500 men in five battalions, but were able to recruit reserves from the 60,000 civilians of the liberated area.

Starting on 10 June, sWest-Bosniens developed as one of the largest, bloodiest and longest anti-partisan operations carried out in Croatia, and came to be seen by the German and Croat high commands as a major success. In the bitter fighting which followed the Axis forces' encirclement of the liberated area, some partisan units managed to break out during the night of 3/4 July, but the main group again came under close attack during the following night and was largely destroyed. In Siroka Luka about 500 wounded partisans were killed. Only small numbers of partisans managed to get away by burying their weapons, disguising themselves as civilian refugees, and passing through the Axis lines.

According to the Germans, the results up to 31 July, which included a two-week period of mopping up after the end of the main fighting on 17 July, the German losses were 69 men killed, 160 wounded and seven missing, the Croat losses 445 men killed, 654 wounded and 498 missing, and the partisan losses 4,310 persons killed and 10,704 taken prisoner. Post-war Yugoslav accounts admitted to only 1,700 killed, wounded and missing from the 2^nd 'Krajina' Assault Brigade, the huge difference in the totals resulting from the Axis forces' listing of several thousand civilian dead as partisan auxiliaries.

Some 75,000 civilian refugees were driven out of their villages in the Kozara area, and many of these villages were then burned to the ground. Looting was common and atrocities were numerous. Of the refugees, 68,000 were forced into transit and detention camps, where 'selections' were then made for slave labour service.

About 900 partisans survived and were re-established as the 5^th 'Krajina' Brigade. At the same time, and under Tito’s personal leadership, the main partisan group moved from eastern to western Bosnia and, after most of the Axis force had been pulled out, recovered parts of lost area in September 1942.