This was a German, Italian and Croat operation against the partisan forces of Josip Broz Tito in the eastern part of the puppet state of Croatia in Axis-occupied Yugoslavia (8 April/14 June 1942).
‘Trio’ (i) followed an initial two Axis offensives which became knows to the partisans as 'First Enemy Offensive' and 'Second Enemy Offensive'.
The 'First Enemy Offensive' (‘Užice’) had taken place in the autumn of 1941 against the ‘Republic of Užice’, a liberated area which the partisans had established in western Serbia. In November 1941, German troops attacked and reoccupied this territory, most of the partisans managing to escape toward Bosnia. It was during this offensive that tenuous collaboration between the communist partisans and royalist Četnik movement collapsed into open hostility. The 'Second Enemy Offensive' (‘Südost Kroatien’ and ‘Ozren’) had been a co-ordinated Axis attack of January 1942 against the partisan forces in eastern Bosnia. The partisans once again managed to avoid encirclement but were compelled to retreat over the Igman mountain near Sarajevo.
As noted above, ‘Trio’ (i) became know to the Yugoslav communist partisans as the 'Third Enemy Offensive', and involved Josip Broz Tito’s partisan forces on one side, Četnik anti-communist irregular forces on the second side, and German, Italian, Ustaše and Croat home guard troops on the third side, and took place in eastern Bosnia, Montenegro, Sandžak and Herzegovina as the Axis forces sought to trap and destroy the 18,000 or so men of the 1st Proletarian Brigade, 2nd Proletarian Brigade, 18 partisan detachments and two independent partisan battalions.
On the Axis side, the force involved was General Paul Bader’s Kampfgruppe ‘Bader’, which comprised from the Germans Generalleutnant Johann Fortner’s 718th Division and the 737th Regiment of Generalleutnant Dr Walter Hinghofer’s 717th Division; from the Italians elements of Generale di Divisione Giovanni Maccario’s 1a Divisione alpina ‘Taurinense’, elements of Generale di Brigata Lazzaro Maurizio de Castiglione’s 5a Divisione alpina ‘Pusteria’, elements of Generale di Divisione Vittorio Ruggero’s 22a Divisione ‘Cacciatore delle Alpi’, 1o Gruppo alpino ‘Alpi Valle’, 2o Gruppo leggero corazzato ‘San Marco’, 12o Gruppo d’artigliera and several armoured car squadrons all supported by Četnik auxiliaries; and from the Croats the 1/13th Regiment, two companies of the 8th Regiment, two companies of the 15th Regiment, elements of the 9th Artillery Group, the 3rd and 4th Military Frontier Battalions, and three battalions of the Ustaše ‘Black Legion’.
The operation had been planned at the inter-Axis command conference held at Opatija on 2/3 March 1942 by representatives of the German, Italian and Croat high and local commands in Yugoslavia. Present at the conference were General Walter Kuntze (representing Generaloberst Alexander Löhr, the Oberbefehlshaber ‘Südost’), Bader (the Militärbefehlshaber in Serbien, or military commander in Serbia), Generalleutnant Enno von Rintelen (German liaison officer with the Comando Supremo, or Italian general staff), Generale d’Armata Vittorio Ambrosio (Italian army chief-of-staff), Generale di Corpo d’Armata Mario Roatta (commander of the Italian 2nd Army), Generale di Divisione Antonio Gandin (representative of the Comando Supremo) and Podmaršal Vladimir Laxa (Croat home defence force chief-of-staff).
Evaluation of the Axis attempts to put down the partisan uprising during the autumn of 1941 and winter of 1941/42 indicated that these had clearly failed, and that the national liberation movement had developed strongly in eastern Bosnia, Montenegro, Bosnian krajina and different parts of Croatia. The conference concluded that the Axis position in the Balkans has been seriously threatened, and that the security of the coastal region and of the vital inland lines of communication was questionable.
Because of that it was decided to launch, during the spring of 1942, a new, altogether larger and for the first time a thoroughly co-ordinated series of operations to destroy the partisan forces in their stronghold areas of eastern Bosnia, Herzegovina, Montenegro and Sandžak, and later in western Bosnia, Croatia and Slovenia. As there were insufficient forces to launch the offensive simultaneously in all of the intended regions, the German and Italian generals agreed to a sequence of operations directed first against east Bosnia, then west Bosnia, Kordun and Banija. While the offensive was being undertaken in east Bosnia, Italian and Četnik forces would undertake operations against partisan elements in Sandžak, Montenegro and Herzegovina.
‘Trio’ (i) was specifically that part of the overall operation in east Bosnia, and achieved only indifferent results. The undertaking was initially schemed as a large operation in eastern Bosnia that was to unfold in three phases beginning with ‘Trio I’ on 15 April to clear the area to the south and east of Sarajevo, then proceeding to ‘Trio II’ to clear the Drina river bend, and ending with ‘Trio III’ to destroy the partisans forces in the Ozren mountain region.
‘Trio I’ began before it was supposed to do so when the Ustaše ‘Black Legion’ became embroiled in heavy fighting with Bosnian Četnik units on 8 April around Drinjača on the Drina river. During the confusion of the next few days, most of the partisans in this area withdrew south toward the Italian zone of occupation, which when appreciated necessitated a complete reappraisal of the operation’s plan. The Italians were unable to bring their forces into play by the date scheduled, so Bader sought authorisation to cancel the entire operation, this permission being granted.
Meanwhile, German and Croat forces from the Sarajevo area reinforced the Ustaše units pushing southward from Drinjača and succeeded in clearing the Drina river bend by 25 April. The 5a Divisione alpina served simply as a blocking force on the southern periphery of the operation.
With the following ‘Foča’, ‘Trio’ was one of the most extended operations to take place in Yugoslavia as it continued in one form or another until 14 June, with most of the fighting after 25 April taking place in Montenegro, Sandjak and Herzegovina between the partisans and Italians in ‘Foča’). According to the Germans, they themselves lost four men killed and 11 wounded, the Croats nine dead and 11 wounded and the Italians three dead and one wounded, while the partisans lost 80 killed and 786 captured. These losses comprise only those reported between 8 and 25 April, and do not include Italian and partisan losses after that date in ‘Foča’.