This was a Croat and German operation against the partisan forces of Josip Broz Tito in the puppet state of Croatia in occupied Yugoslavia (27 February/8 March 1943).
The operation’s task was the destruction of the concentrations of Izidor Strok’s 400-man 'Kalnik' Partisan Detachment in the Kalnik and Bilo mountain region along the railway linking Krizevci and Koprivnica, and for this task the Croats and Germans mustered some 2,300 men including one unidentified company of Germans and a largely Croat grouping controlled by the headquarters of Pukovnik Matija Murković's 1st Division and divided into two groups. The main attacking force was Pukovnik Anton Nardeli’s Group A, which comprised the staff of the 11th Regiment, Ustase Major Josić's Provisional Battalion 'Ivan' (using men of the 1st Expeditionary Company/2nd Regiment, 1st Recruit Squadron/Cavalry and 1st Battery/2nd Artillery Group), Satnik Josip Horzić's Provisional Battalion 'Josip' (using men of the 1st Bicycle Battalion, 1st Battery/1st Artillery Group and 2nd Battery/2nd Artillery Group), Bojnik Miroslav Stampar’s Provisional Battalion 'Antun' (using men from the Legionnaire Company/1st Regiment, Assault Company/1st Cavalry Group and a Ustase company).
Tasked with closing on the south-eastern side of Bilo mountain to prevent the escape of partisans in the direction of the Grubizno Polje region, Podpukovnik Josip Trupac’s Group B comprised the 2nd Expeditionary Company/1st Regiment, 14th Company/1st Regiment, and one company of the Bjelovar Gendarmerie School.
The partisans received a warning about the operation about one day before its launch, and this reinforced a decision their command had already made for the detachment to leave Kalnik mountain, although only on a temporary basis, and link with the 'Moslavina Partisan Detachment. Using the information they had received, the partisans were able to establish several ambushes. The Provisional Battalion 'Ivan', which was to have arrive at 03.00 on 27 February at Krizevci after railway movement from from Zagreb, was 90 minutes late, and thus failed to catch the partisan rearguard, which had sufficient time to retreat. On the following day the battalion commander, Josić, made little effort to make up for the time lost on the previous day and the battalion failed to gain ground on the partisans. During the afternoon of 28 February an ambush was set and sprung near the village of Gornji Tkalec for the German motorised company which had been despatched to intercept them and thus prevent their escape.
After the success of its ambush, the 'Kalnik' Partisan Detachment withdraw once darkness had fallen, moving toward Gradec and destroying telegraph equipment at the railway stations along its route. Once it had been appreciated that the Croat force’s lack of mobility was making it impossible to overtake the partisans, the dismounted men of the 2nd Bicycle Company were sent back to Koprivnica for their bicycles and moved to Krizevci, where they reinforced the German motorised company. On 1 March there arrived a reinforcement from Zagreb in the form of Ustaze Bojnik Oresković's 1st Armoured Car Company of Ustase Pukovnik Ante Moskovss Poglavnik Bodyguard Brigade, and this was to prove vital in the encirclement.
The 'Kalnik' Partisan Detachment rested near the village of Bojana, and here a peasant from Miklusevo provided warning of the Croat reinforcement, with tanks and trucks. Forced to retreat, the 'Kalnik' Partisan Detachment retired in some disorder into the Pijesak forest, where it was surrounded and broken. After a failed attempt to break out, the detachment’s staff and 1st and 2nd Battalions managed to slip out during the night toward Samarica and Podgarić, in the process losing contact with the 3rd Battalion as this escaped toward Bilo mountain. The Croat troops captured the wagons carrying the partisan wounded.
With the Kalnik area cleared of partisans, a sweep operation was carried out on 5/8 March in an attempt to eliminate any remaining smaller groups, but this did not yield significant results. Starting from Koprivnica, Ludbreg, Krizevci and Varazdin, the Croat columns had combed the Kalnik mountain region for a little over a week, but the partisans had evaded them and the results were poor. The Germans lost three men killed and 12 wounded, the Croats suffered no casualties, and the partisans suffered 49 killed, 20 wounded and 16 captured.