Third anti-Partisan Offensive

Operation Trio
Part of World War II in Yugoslavia
Tito and the main force of the Partisans withdrew into the Zelengora ("green hill") mountains to escape the Axis offensive
Date 20 April – 13 May 1942 Location Eastern Bosnia Result See the Aftermath section
Belligerents
Germany  Italy NDH	Chetniks (East Bosnian)	Partisans
Commanders and leaders
Paul Bader Mario Roatta Jure Francetić	Stevan Botić Dragoslav Račić Milorad Momčilović	Josip Broz Tito Konstantin Popović
Units involved
718th Infantry Division 22nd Infantry Division 1st Alpine Division 5th Alpine Division 3 Black Legion battalions 25 Croatian Home Guard battalions	Unknown	1st Proletarian Brigade 2nd Proletarian Brigade 1st East Bosnian Shock Battalion 2nd East Bosnian Shock Battalion East Bosnian Partisan Detachments
Strength
	around 4,000 men
Casualties and losses
Italians 220 killed 556 wounded 173 missing Germans 11 killed 15 wounded 1 missing NDH forces 82 killed 149 wounded 121 missing Chetniks Unknown	Chetniks 74 killed 102 wounded 3 missing	Partisans 1,646 killed 719 wounded 2,626 captured (259 executed) large number of defectors (to the Chetniks)

Operation Trio (Serbo-Croatian: 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 groups led by communists and Serb nationalist Chetniks. Differentiating between the 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: 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.

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