[1] The majority of the units of the National Liberation Army (1st and 2nd Proletarian, 3rd Sandžak and 4th Montenegrin Brigades) and the Supreme Headquarters of the NOV and POJ set out from the territory of Mt Zelengora on June 24, 1942, for western Bosnia.

In the first stage, a sudden attack was to break and destroy enemy crews and destroy the Sarajevo-Mostar railway, and then, in the next stage, continue to advance to the northwest, take control of the territory on the right bank of the Neretva, in the upper Vrbas and Kupres, Livno, Imotski and connect with Krajina and Dalmatian units.

During the first six months of 1942, the partisans suffered heavy losses in Second and Third Enemy Offensives in eastern Bosnia, Herzegovina, Sandžak and Montenegro.

By the joint action of the 2nd Proletarian Brigade, Herzegovinian and Montenegrin partisans, Borač, a strong Ustashe stronghold, near Kalinovik, was liberated.

[4] The insurgent forces in Grmeč and Kozara increased especially, and on May 16, 1942, the 1st and 2nd Krajina detachments liberated Prijedor, which was defended by about 2,000 enemy soldiers.

In order to prevent the further spread of the uprising, Germans and Italians, on 3 March 1942, in Opatija, signed an agreement which reached a plan for a general offensive against the partisans.

Decision of Partisan Supreme HQ to move west proved to be very prudent for a number of reasons, the most important of which was the fact that the Italians, on the basis of the Zagreb agreement concluded on June 19, withdrew a large part of their troops from zones II and III.

After managing to break up the Ustasha forces around the Treskavica mountain, on 3 July 1942, partisan brigades carried out a surprise attack on the Sarajevo-Konjic railway between Hadžići and Konjic.

This was a major blow to the occupying forces and the NDH, because ties with Herzegovina were severed for more than two months and the transport of bauxite from the Mostar region was prevented.

Although in autumn 1942 Partisans would be forced to leave Prozor to the Italians, Livno to the Croatian quisling forces, and Jajce to the Germans, the partisans nevertheless controlled the area from the western approaches to the Neretva in the south to Karlovac in the north, an area of 250 km in length and 40 – 70 km in width.