'Trio' (i) was the first large-scale joint German, Italian and Croat counter-insurgency operation of World War II and was undertaken conducted in the puppet state of Croatia, which then included modern-day Bosnia and Herzegovina (20 April/13 May 1942).

The Axis operation was effected in two phases within eastern Bosnia, and involved on the Axis side Ustase militia and Croat army forces. The undertaking’s object was the targeting of major insurgent forces between Sarajevo and the Drina river, and these included the communist-led Yugoslav partisans and the Serb nationalist Četniks. Any real differentiation between the rank and file of the two insurgent groupings was difficult for even the partisans comprised primarily Serb peasants who had little understanding of the political aims of their leaders.

'Trio' (i) comprised 'Trio I' and 'Trio II', which together were a single element of the Axis effort known that became known in post-war Yugoslav history as the 'Third Enemy Offensive', whose other major component was the joint Italian and Četnik offensive in Montenegro and eastern Herzegovina.

The operation achieved only limited success as a result of several factors including pre-emptive action by the Ustase militia and Italian delays. The area of operations straddled the demarcation line between the German and Italian zones of occupation within Croatia, which led to mutual suspicion and lack of co-ordination. Both insurgent factions avoided fighting the Axis and Croat forces, but instead focused their efforts on fighting each other. After the end of 'Trio', the partisan leader Josip Broz Tito, his headquarters and the partisan main force, comprising the 1^st and 2^nd Proletarian Brigades, withdrew from their base of operations around Foça. After briefly reorganising in the Zelengora mountain area, to the south-east of Foça, they shifted the weight of their operations to western Bosnia for the remainder of 1942.

'Trio' (i) coincided with and in fact made a signal contribution to the polarisation of the almost exclusively Serb insurgents in eastern Bosnia into two groups: the Serb-based chauvinist Četniks and the multi-ethnic and communist-led partisans. Encouraged by Četnik propaganda against the Croats and the Bosnian moslems, and repelled by the sectarian left-wing policies and actions of the communists, many Serb peasant fighters were swayed to the Četnik cause. There were violent coups occurred against the communist leadership of all but one of the partisan detachments in eastern Bosnia, and these detachments in effect defected to the Četniks. Most of the surviving communist fighters from these detachments rejoined the partisan forces, and many withdrew with Tito in the 'long march' into western Bosnia. Within a few weeks of the end of 'Trio' (i), a mere 600 partisan fighters were left in eastern Bosnia, these comprising the Group of Shock Battalions and the 'Birač' Partisan Detachment, which attempted to find refuge in the Birač region. The Četnik movement in eastern Bosnia, which was in reality little more than confederacy of local warlords, was strengthened by mass defections from the partisans, and for a time ruled large parts of the region after reaching accommodations with the Ustase movement in May and June 1942.

In the course of 'Südwest Kroatien', Tito, his headquarters and Pukovnik Konstantin 'Koča' Popović's 1^st Proletarian Brigade had withdrawn to the south to Foča, on the boundary between eastern Bosnia and Herzegovina. With the help of Montenegrin partisans, there they created a liberated area around Foča and Gorazde known as the 'Foča Republic', which was expanded by subsequent military operations. By a time late in March, so-called people’s liberation councils had been established to govern 10 towns and 92 villages in the liberated area, though communist organisation in the area was both limited and of poor quality.

At the end of 1941 there were six partisan detachments in eastern Bosnia with about 7,300 fighters with which to operate in the Majevica, Ozren, Birač, Romanija, Zvijezda and Kalinovik areas. In January 1942, the 'Romanija' Partisan Detachment had borne the brunt of 'Südwest Kroatien', in which it had been effectively destroyed. Many of the partisan fighters were Serb peasants who took to the forests and mountains to defend their families and villages against the Ustase, and only a comparatively small number were ideologically committed to the partisan cause. The Četnik forces in eastern Bosnia had not opposed the Axis offensive, many of them pulling back across the Drina river into the territory of the German Befehlshaber Serbien to avoid engagement with German and Croat forces.

Both the partisan supreme headquarters and the partisan general staff of Bosnia-Herzegovina were based in the area of operations, with Tito’s supreme headquarters inb direct command of the 1^st Proletarian Brigade and the General Staff, commanded by Pukovnik Svetozar Vukmanović-Tempo’s general staff controlling the partisan detachments in East Bosnia under the overall supervision of the supreme headquarters.

Early in January 1942, the partisan supreme headquarters decided to permit fighters who were unwilling to make a complete commitment to the partisan cause to fight alongside partisan units. These 'volunteer detachments' were under the control of the supreme headquarters of what was now known as the People’s Liberation Partisan and Volunteer Army of Yugoslavia, and were established from former Četnik-aligned fighters as the 'Jahorina', 'Foča', 'Vlasenica' 'Srebrenica' and 'Krajina' Volunteer Detachments. The last comprised refugees from that region who had fled to German-occupied Serbia to escape the Ustase terror. Volunteer battalions and companies were also placed under the staff of the original partisan detachments, many of them being absorbed as whole units with the addition of a communist cadre. Some volunteer detachments fought under their own leaders, and all volunteer detachments fought under the Serb tri-colour flag.

In February 1942, Major Jezdimir Dangić and other officers of the former Yugoslav army, many of whom had allegiance to the Serb puppet regime of Milan Nedić and/or 'Draza' Mihailović, entered eastern Bosnia from occupied Serbia, where some of them had withdrawn to avoid 'Südost Kroatien'. Here they began to re-form Četnik units in eastern Bosnia and began agitating against the partisans on what could be described as a conservative, Serb nationalist and anti-moslem platform. Other Četnik units crossed into eastern Bosnia from occupied Serbia and attacked the partisans: these units included the Četnik Proletarian Shock Brigade, which was a unit of 200 fighters under the command of Kapetan Dragoslav Račić, and another group under the command of Kapetan Milorad Momčilović.

The partisan forces in eastern Bosnia and Herzegovina were initially based almost entirely ob Serb peasants, and this made much of the rank and file of both the partisan and the volunteer detachments very susceptible to pro-Četnik agitation, accommodations with Četnik forces in their areas, and hostility toward those of non-Serb ethnic nature. The partisan moves toward multi-ethnic recruiting, the imposition of extreme left-wing policies and the use of terror against 'class enemies' made all the partisan and volunteer detachments vulnerable to such agitation. Četnik infiltrators were able to join detachments and turn the rank and file against their communist cadres. An example of this occurred in the 'Majevica' Partisan Detachment on 20 February, when the communist staff personnel were massacred by Četniks at Vukosavci.

The 2^nd Proletarian Brigade was formed at Čajniče during 1 March on the basis of partisan forces which had withdrawn from occupied Serbia after 'Uzice'. Early in March the partisans began to group the most loyal fighters from each partisan detachment into shock companies and established structures for the development of larger 'shock battalions' and 'shock brigades'. At the same time, partisan forces which had been dispersed by 'Südost Kroatien' were starting to threaten the railway line linking Tuzla and Doboj. In the middle of March the 1^st 'East Bosnian' Shock Battalion was established at Srednje outside Sarajevo, and by the end of the month the 2^nd 'East Bosnian' Shock Battalion had been established in Drinjača (near Zvornik. It incorporated the remaining 240 fighters of the 'Majevica' Partisan Detachment.

However, the concentration of the most reliable fighters into proletarian brigades, shock battalions and shock companies weakened the integrity of the four remaining partisan detachments in eastern Bosnia, but on the other hand made it possible for the partisan supreme headquarters to concentrate its most capable forces in mobile units to undertake successful offensive operations against the Četniks. During March, mobile units of this nature captured several towns including Vlasenica and Srebrenica, and by the end of the month partisan operations had become a threat to the railway network throughout eastern Bosnia, including that in the area of Sarajevo. Many Bosnian Četniks deserted to the partisan movement, often joining as complete units under their previous Četnik commanders. These former Četnik units became units of the 'Volunteer Army', which reached a strength of between 7,000 and 8,000 fighters by the end of March, though their loyalty and military value to the partisan movement was an indeed remained very limited.

On 25 March the partisan general staff of Serbia advised the partisan supreme headquarters that the Serb partisan movement had been extinguished, largely as a result of 'Uzice' and following operations by the German occupation forces and their Serb collaborators. This was a significant setback for the partisan cause as Tito had always considered that a return to Serbia was an factor necessary for a successful revolution.

Planning for 'Trio' (i) and the associated 'West Bosnien' in the Bosanska Krajina area took place at a pair of Axis conferences in March. The first of these conferences, at Opatija on 2/3 March, was attended by General Walter Kuntze (representing Generaloberst Alexander Löhr, the Oberbefehlshaber 'Südost'), Bader (the Militärbefehlshaber '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 2a Armata), Generale di Divisione Antonio Gandin (representative of the Comando Supremo) and Podmarsal Vladimir Laxa (Croat army chief-of-staff). The last raised objections to an Italian proposal to involve the Bosnian and Herzegovinian Četniks in the planned operations and, with German support, the idea was initially shelved. 'Trio' (i) was to be one of a series of counter-insurgency operations planned for areas of eastern Bosnia, Herzegovina, Sand≈æak, Montenegro, western Bosnia and Lika. Despite this, the only operations that came to fruition between March and June 1942 were 'Trio' (i) and a combined Italian and Montenegrin Četnik offensive in Montenegro and eastern Herzegovina: it was these two Axis undertaking which became known to the Yugoslavs as the 'Third Enemy Offensive'.

The detailed planning and orders for 'Trio' (i) were finalised at a conference in Ljubljana on 28/29 March 1942. Laxa, Roatta and General Paul Bader, commander of the German forces in Croatia, negotiated a compromise permitting temporary non-political agreements to be concluded with the Herzegovinian Četnik forces of the self-styled 'Vojvoda' Dobroslav Jevđević, but not with any of the Bosnian Četnik groups, whose leaders were Major Petar Baćović in the area of Foča and Jezdimir Dangić, who was aligned with the Serb collaborationist Milan Nedić.

Significant delays in finalising the arrangements for 'Trio' (i) arose from disagreements regarding where it would start, who was to be in command, the involvement of Četnik and Croat forces, how the demarcation line between the German and Italian zones of occupation was to be handled, and which local authorities would be installed at places cleared of insurgents, and th whole undertaking was bedevilled by the mutual suspicion between the Germans and Italians. In the end it was the Italian demands that prevailed as it was they who were committing larger forces. The decision was ultimately made to target all the insurgent forces in eastern Bosnia between Sarajevo and the Drina river. Throughout the preparation for 'Trio' (i), the Italians looked for opportunities to cross over the demarcation line and expand their sphere of influence into eastern Bosnia and thus take advantage of German weakness in Croatia. Bader’s final orders for the operation conceded several key Italian demands, including military control over civil affairs in the area of operations, fair treatment of the local population, and the treatment of non-resisting Četniks as prisoners of war.

Bader was nominated as the tactical commander of the combined forces, known as the Kampfgruppe 'Bader' committed to 'Trio' (i), but to appease the Italians the force was formally under the overall command of the Italian 2a Armata. The Kampfgruppe 'Bader' comprised, from the Germans, Generalleutnant Johann Fortner’s 718^th Division and the 737^th Infanterieregiment of Generalleutnant Dr Walter Hinghofer’s 717^th 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', the 1o Gruppo alpino 'Alpi Valle', the 2o Gruppo leggero corazzato 'San Marco', the 12o Gruppo d’artigliera and several armoured car squadrons supported by Četnik auxiliaries; and from the Croats, the 1/13^th Regiment, two companies of the 8^th Regiment, two companies of the 15^th Regiment, elements of the 9^th Artillery Group, the 3^rd and 4^th Military Frontier Battalions, and three battalions of the Ustase 'Black Legion'.

Since 18 February, the 718^th Division, which was then the only complete German division stationed in Croatia, had been responsible for an operational area bounded by the Sava and Bosna rivers in the north, the Drina river to the east, and the German/Italian demarcation line to the south. As a result mainly of its lack of transport and firepower, the division had undertaken only limited offensive operations against the partisans between the middle of February and the middle of April.

The originally planned start date of 15 April was pushed back when the Italians had problems moving forces to their start positions and later had trouble providing transport to establish secure lines of communication across the Adriatic Sea. The operation was therefore rescheduled to begin on 25 April. Before the Ljubljana conference, the Ustase authorities were concerned about negotiations between the German and Italian commanders and Dangić, and were particularly worried that the Germans would permit the Italian and Četnik forces to use Sarajevo as a base. On 31 March, the commander of the Ustase 'Black Legion', Pukovnik Jure Francetić, launched a pre-emptive offensive primarily against Dangić's Četniks. Francetić's forces captured Vlasenica, Bratunac and Srebrenica in the face of only limited resistance from the partisans, and then scattered the more numerous Četniks while inflicting significant losses.

Early in April, Dangić travelled to Belgrade for discussions with representatives of Nedić and of the Četnik leadership. On arrival, he was arrested by the German authorities and sent to a prisoner of war camp in occupied Poland. Dangić was replaced by Stevan Botić. On 15 April, the Wehrmachtbefehlshaber Südost, General Walter Kuntze, issued an order forbidding German units from entering negotiation with any insurgent groups. Only Abwehr military intelligence and police units were to maintain surveillance of such groups through informants and undercover agents.

Following several months of worsening tensions between the factions struggling for power within the insurgency, the first of the pro-Četnik coups took place, in this instance in the 'Ozren' Partisan Detachment. It was sparked by the arrest and execution on 18 April of the pro-Četnik agitator Bogdan Jovićić by Vukmanović-Tempo and the newly-established 1^st 'East Bosnian' Shock Battalion. Fighting then followed between the detachment’s pro-Četnik members and the shock battalion. Vukmanović-Tempo then abandoned the 'Ozren' Partisan Detachment, taking the detachment staff and remaining loyal partisans with him.

On 18 April, Bader urged on Roatta of the need to take action immediately in order to relieve the Croat garrison currently under siege at Rogatica, and by 20 April Bader was advising his superiors that the joint German and Italian operation had miscarried as a result of Italian inaction. In the wake of Francetić's offensive, the Germans moved pre-emptively to clear the area to the north of the demarcation line before the formal start of the operation. This advance toward the Drina river, between 20 and 30 April, was co-ordinated with Croat forces and was the first phase of 'Trio' (i), which soon became 'Trio I'. The 718^th Division advanced from assembly areas in Sarajevo, Olovo and Tuzla, with the aim of relieving Rogatica and clearing the surrounding area of partisans. The fighting became very confused, with the Četniks, who were under attack from the Ustase 'Black Legion', avoiding the German units, who bypassed them to attack the partisans. The partisan main force avoided fighting the Ustase 'Black Legion', instead attacking the Četniks from the rear while they were engaged against Francetić's troops. The Italian 5a Divisione alpina 'Pusteria' made use of Četnik troops from the Sandzak area as auxiliaries during their advance on Čajniče, which coincided with the German and Croat advance toward the Drina river. Rogatica was relieved without fighting on 27 April, and the combined force reached the Drina river three days later.

More pro-Četnik coups occurred during the second half of April. The first took place in one of the remaining battalions of the 'Romanija' Partisan Detachment, followed by all three battalions of the 'Zvijezda' Partisan Detachment: in these the political commissars of every company were killed. By the start of May, coups had also occurred in battalions of the 'Kalinovik' Partisan Detachment and the 'Foča' Volunteer Detachment. On 8/9 May, another pro-Četnik coup occurred in the recently created 'Zenica' Partisan Detachment, and about 30 communists and their supporters were killed. About 100 remaining fighters from the 'Ozren' Partisan Detachment and 'Zenica' Partisan Detachment were then incorporated into the 3^rd 'East Bosnian' Shock Battalion.

The Italians believed the German and Croat preliminary operation had been designed to avoid the need to involve the Italians in clearing eastern Bosnia, thereby preventing them from expanding their sphere of influence. The second phase of the operation was 'Trio II' (otherwise 'Foča') and this began on 7 May as a fairly minor joint operation to capture Foča and Kalinovik, but by then the partisan supreme headquarters and main force had already evacuated Foča, which was captured on 10 May. After Italian complaints and political manoeuvrings, Roatta assumed direct control of the operation on that day, but the fighting was already over. Despite their attempts to avoid fighting, the partisans suffered significant losses.

After clearing the larger towns of the Birač region of partisans and Četniks, the Ustase 'Black Legion' committed major atrocities against Serbs and Jews in the region, these atrocities including the massacre of about 890 people from Vlasenica after raping the women and girls.

Along with the three 'East Bosnian' Shock Battalions, the partisan general staff of Bosnia-Herzegovina first attempted to cross the Bosna river to follow the partisan supreme headquarters and main force to western Bosnia, but instead retreated to Birač, where it joined the 'Birač' Partisan Detachment at the end of May. The 'Birač' Partisan Detachment was the only partisan or volunteer detachment in East Bosnia not to suffer a pro-Četnik coup in the period between March and May 1942, but by the period between June and July the partisan forces in eastern Bosnia had been reduced to a strength of about 600 fighters.

In the middle of May, 'Trio' (i) was followed by the joint Italian and Četnik offensive against partisan detachments within the Italian zone of occupation in eastern Herzegovina and Montenegro, with similar effects: the partisans lost almost all of the territory they had liberated in these areas. This offensive is also considered part of the 'Third Enemy Offensive' in Yugoslav history. After 'Trio' (i), Croat forces remained south of the demarcation line between the German and Italian zones of occupation despite the protests of the Italians.

After 'Trio' (i) and the joint Italian and Čhetnik offensive, the partisans formed three more proletarian brigades, consisting mainly of Montenegrins. 'Trio' (i) contributed to the decision of the partisan supreme headquarters to withdraw to western Bosnia in its 'long march', which started late in June 1942.

While it had suffered major losses as it fought the Ustase 'Black Legion', the Četnik movement in eastern Bosnia did derive some benefit from the mass desertion of partisans and the many pro-Četnik coups in partisan and volunteer detachments. Despite their lack of unity, the Četnik movement thrived in eastern Bosnia for the remainder of 1942 because some Četnik leaders reached accommodations with the Ustase régime and as many Četniks and partisans were unwilling to kill fellow Bosnian Serbs of the opposing faction’s militia and Croat army forces. The undertaking’s object was the targeting of major insurgent forces between Sarajevo and the Drina river, and these included the communist-led Yugoslav partisans and the Serb nationalist Četniks. Any real differentiation between the rank and file of the two insurgent groupings was difficult for even the partisans comprised primarily Serb peasants who had little understanding of the political aims of their leaders.