Syrmian Front The Syrmian Front ( , Сремски фронт ) was an Axis line of defense during World War II . It was established as part of the Eastern Front in late October 1944 in Syrmia and east Slavonia , northwest of Belgrade . After the Yugoslav Partisans and the Red Army expelled the Germans from Belgrade in the Belgrade Offensive , the retreating Wehrmacht and the Croatian Armed Forces used fortifications to protect the withdrawal of German Army Group E from the Balkans . With help from their Soviet allies , the Partisans ( by then recognized as the Yugoslav army ) , joined by Bulgarian and Italian forces , fought a difficult winter campaign and finally broke through the front on 12 April 1945 . After the Syrmian front was broken , occupied Yugoslavia was liberated . Operative background and significance . After the September advance through Romania and Bulgaria in October 1944 , The Red Army , together with Yugoslav forces , took Belgrade ( central communication node of the Balkans ) in the Belgrade Offensive . Due to Yugoslav partisan activity , the Yugoslav - Allied Operation Ratweek , and pressure from the Bulgarian Army , the Germans failed to prevent this while they awaited the redeployment of Army Group E troops from Greece . The Red Army decided to exploit this delay and continued to advance with the 3rd Ukrainian Front from Belgrade to south - west Hungary . The aim of the advance was to separate and protect their main attack in Hungary from attacks on the flank by Army Group E from the south . From September 1944 to January 1945 , Army Group E pushed its way through Macedonia , Kosovo , Sanjak , and Bosnia , and soon their sole available escape route was in a line between Sarajevo and Slavonski Brod . For this reason , it was of vital significance for the Germans to defend the zone around Slavonski Brod , which was threatened by the Soviet - Yugoslav advance through Syrmia . To prevent Army Group E from being cut off , the German South - East command prepared seven successive fortified defense lines between the Danube and Sava river from Ruma to Vinkovci . The Syrmian Front campaign consisted of Yugoslavian attempts to break through these lines of defense . Operations . The Syrmian Front saw some of the most difficult fighting in Yugoslavia in World War II . It lasted for almost six months . As the bulk of the Red Army involved in the Belgrade operation continued their offensive in Hungary , the Yugoslav Army , accustomed to guerrilla warfare in the mountainous terrain of the Dinaric Alps , remained to fight the entrenched front line heavily contested by the Axis on the flat ground of the Pannonian plain . Young men from Vojvodina and Central Serbia , many from freshly liberated regions , were drafted " en masse " and sent to the front , and the amount of training they received and their casualty levels remain in dispute . Although mostly stationary , the front moved several times , generally westward , as the Axis forces were pushed back . The fighting started east of Ruma and stabilized in January 1945 west of Šid after the town changed hands due to Axis counterattacks . In late March and early April 1945 , Yugoslav Army units mounted a general offensive on all fronts . The Yugoslav First Army , commanded by Peko Dapčević , broke through German XXXIV Corps defenses in Syrmia on 12 April , quickly capturing the cities of Vukovar , Vinkovci , and Županja , and enabling further advances through Slavonia toward Slavonski Brod and Zagreb in the last month of the war . The campaign can be divided into four distinct phases : Mobilization motives . According to Serbian historian Srđan Cvetković , the partisans used the front as a way to undertake reprisals against individuals perceived to be collaborators or identified as " enemies of the people " ( those holding values opposed to Communism ) , which resulted in the deaths of thousands of young bourgeois men from Belgrade and other recently captured Serbian cities . The men were sent to the front rather than explicitly liquidated .