Operation Trio (Serbo-Croatian Latin: Operacija Trio) was the first large-scale joint German-Italian counter-insurgency operation of World War II conducted in the Independent State of Croatia (NDH), which included modern-day Bosnia and Herzegovina. 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. The aim of the operation was to target all insurgents between Sarajevo and the Drina river in eastern Bosnia. These included the communist-led Yugoslav Partisans and Serb nationalist Chetniks. Differentiating between the rank and file of the two insurgent factions was difficult, as even the communist-led insurgent groups consisted mainly of Serb peasants who had little understanding of the political aims of their leaders.

Operation Trio consisted of two parts, Trio I and Trio II. 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 joint Italian-Chetnik offensive in Montenegro and eastern Herzegovina formed the other element. The Third Enemy Offensive forms part of the Seven Enemy Offensives framework in Yugoslav historiography.

The operation was of limited effectiveness due to several factors, including preemptive action by the Ustaše militia and Italian delays. 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. Both insurgent factions avoided fighting the Axis and NDH forces, instead focusing on fighting each other. After Operation Trio, the Partisan leader Josip Broz Tito, his Supreme Headquarters and the Partisan main force, consisting of the 1st and 2nd proletarian brigades, withdrew from their base of operations around Foča. After briefly reorganising around Zelengora mountain south-east of Foča, they moved their operations to western Bosnia for the remainder of 1942.

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. Encouraged by Chetnik propaganda against Croats and Bosnian Muslims and repelled by the sectarian left-wing policies and actions of the communists, many Serb peasant fighters were swayed to the Chetnik cause. Violent coups occurred against the communist leadership of all but one of the Partisan detachments in eastern Bosnia, and these detachments effectively defected to the Chetniks. Most of the surviving communist fighters from these detachments rejoined the Partisan forces, and many withdrew with Tito to western Bosnia during the Partisan Long March. Within a few weeks of the end of Operation Trio only 600 Partisan fighters were left in eastern Bosnia, comprising the Group of Shock Battalions and the Birač Partisan Detachment. All these forces sought refuge in the Birač region. The Chetnik movement in eastern Bosnia, at best a confederacy of local warlords, was strengthened by mass defections from the Partisans. For a time they ruled large parts of the region, after making accommodations with the Ustaše regime in May and June 1942.

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. With the help of Montenegrin Partisans, they established a liberated area around Foča and Goražde. This area, known as the "Foča Republic", was expanded by subsequent military operations. By late March, People's Liberation Councils had been established to govern 10 towns and 92 villages in the liberated area, but communist organisation in the area was limited and of poor quality.

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. In January 1942, the Romanija detachment had borne the brunt of Operation Southeast Croatia and had been effectively destroyed. Many Partisan fighters were Serb peasants who took to the forests and mountains to defend their families and villages against the Ustaše; few were ideologically committed to the Partisan cause. The Chetnik forces in eastern Bosnia had not opposed the Axis offensive. Many had withdrawn across the Drina river into the Territory of the Military Commander in Serbia to avoid engagement with German and NDH 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 directly controlling the 1st Proletarian Brigade, and the general staff, commanded by Svetozar Vukmanović-Tempo, controlling the Partisan detachments in East Bosnia under the overall direction of the supreme headquarters.

In early January 1942, the Partisan Supreme Headquarters decided to permit fighters who were not willing to formally become Partisans to fight alongside Partisan units. These "volunteer detachments" were under the control of the supreme headquarters of the renamed People's Liberation Partisan and Volunteer Army of Yugoslavia, and were established from former Chetnik-aligned fighters as the Jahorina, Foča, Vlasenica, Srebrenica and Krajina volunteer detachments. 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. Some volunteer detachments fought under their own leaders, and all volunteer detachments fought under the Serbian tri-colour flag.