It was carried out in two phases within eastern Bosnia from 20 April to 13 May 1942, with Ustaše militia and Croatian Home Guard forces taking part on the Axis side.

Together they comprised one element of the Axis effort known as the Third Enemy Offensive (Serbo-Croatian Latin: Treća neprijateljska ofenziva) in post-war Yugoslav historiography.

The area of operations straddled the demarcation line between the German and Italian zones of occupation within the NDH, which led to mutual suspicion and lack of coordination.

Operation Trio coincided with and contributed to the polarisation of the almost exclusively Serb rebels in eastern Bosnia into two groups: the Serb-chauvinist Chetniks and the multi-ethnic and communist-led Partisans.

During Operation Southeast Croatia, Josip Broz Tito, his Supreme Headquarters and the 1st Proletarian Brigade commanded by Spanish Civil War veteran Konstantin "Koča" Popović, had withdrawn south to Foča, on the boundary between eastern Bosnia and Herzegovina.

[5] At the end of 1941, there were six Partisan detachments in eastern Bosnia, with about 7,300 fighters operating in the Majevica, Ozren, Birač, Romanija, Zvijezda and Kalinovik areas.

The Krajina Volunteer Detachment consisted of refugees from that region who had fled to German-occupied Serbia to escape the Ustaše terror.

Volunteer battalions and companies were also placed under the staff of the original Partisan detachments, with many of them absorbed as whole units with the addition of a communist cadre.

[11] In February 1942, Major Jezdimir Dangić and other former Royal Yugoslav Army officers (many of whom had allegiance to the Serbian puppet regime of Milan Nedić or Draža Mihailović) entered eastern Bosnia from occupied Serbia, where some of them had withdrawn to avoid Operation Southeast Croatia.

They started to re-form Chetnik units in eastern Bosnia and began agitating against the Partisans on a "conservative, Serb-nationalist and anti-Muslim basis".

[14] The 2nd Proletarian Brigade was formed at Čajniče on 1 March from Partisan forces that had withdrawn from occupied Serbia after Operation Uzice.

[18] 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 enabled the Partisan Supreme Headquarters to concentrate its best forces in mobile units to undertake successful offensive operations against the Chetniks.

Laxa, General Mario Roatta (the commander of the Italian Second Army), and General der Artillerie Paul Bader (the commander of German forces in the NDH) negotiated a compromise permitting temporary non-political agreements to be concluded with the Herzegovinian Chetniks, led by Dobroslav Jevđević, but not with any of the Bosnian Chetnik groups, whose leaders were Petar Baćović in the area of Foča and Jezdimir Dangić, who was aligned with the Serb collaborationist Milan Nedić.

Throughout the preparation for Operation Trio, the Italians looked for opportunities to cross over the demarcation line and expand their sphere of influence into eastern Bosnia to take advantage of German weakness in the NDH.

[27][28] Since 18 February, the 718th Infantry Division had been responsible for an area of operations bounded by the Sava and Bosna in the north, the Drina to the east and the German-Italian demarcation line to the south.

Mainly because of lack of transport and firepower, the division had only conducted limited offensive operations against the Partisans between mid-February and mid-April.

[30] On 31 March the commander of the Ustaše Black Legion, Jure Francetić, launched a pre-emptive offensive primarily against Dangić's Chetniks.

Francetić captured Vlasenica, Bratunac and Srebrenica, meeting limited resistance from the Partisans, and then scattered the more numerous Chetniks[31] while inflicting significant losses.

[34] After several months of increasing tension between the factions struggling for power within the insurgency, the first of the pro-Chetnik coups occurred, in the Ozren Partisan Detachment.

It was sparked by the arrest and execution on 18 April of pro-Chetnik agitator Bogdan Jovićić by Vukmanović-Tempo and the newly formed 1st East Bosnian Shock Battalion.

[36] In the wake of Francetić's offensive, the Germans pre-emptively moved to clear the area north of the demarcation line before the formal start of the operation.

The Partisan main force avoided fighting the Black Legion, instead attacking the Chetniks from the rear while they were engaged against Francetić's troops.

[44] After Operation Trio and the joint Italian-Chetnik offensive, the Partisans formed three more proletarian brigades, consisting mainly of Montenegrins.