1st Army (Kingdom of Yugoslavia)

The 1st Army faced the Hungarian 3rd Army, and during the first few days after the commencement of the invasion, there were exchanges of fire with Hungarian border guards, but the 1st Army faced no direct attacks. Neither the 1st Army or the Hungarians were ready for full-scale fighting, as they were still mobilising and deploying their forces.[14] On 9 April, due to events in other parts of Yugoslavia, the 6th Army on the right flank of the 1st Army was ordered to withdraw south of the Danube and deploy on a line facing east to defend against an attack from the direction of Sofia, Bulgaria. 2nd Army Group also received orders to withdraw south of the line of the Drava and Danube. 1st Army began to withdraw, and on the same day elements were approaching the Danube crossing.[17]

The following day, the situation deteriorated significantly when the German XLI Motorised Corps crossed the Yugoslav-Romanian border into the Yugoslav Banat and struck the 6th Army, halting its withdrawal and disrupting its ability to organise a coherent defence behind the Danube.[18] Also on 10 April, the main thrust of the XLVI Motorised Corps of the 2nd Army, consisting of the 8th Panzer Division leading the 16th Motorised Infantry Division crossed the Drava at Barcs in the 4th Army sector. The 8th Panzer Division turned southeast between the Drava and Sava rivers, and meeting almost no resistance and with strong air support, had reached Slatina by evening, despite poor roads and bad weather.[19]

Later that day, as the situation was becoming increasingly desperate throughout the country, Dušan Simović, who was both the Prime Minister and Yugoslav Chief of the General Staff, broadcast the following message:[19]

All troops must engage the enemy wherever encountered and with every means at their disposal. Don't wait for direct orders from above, but act on your own and be guided by your judgement, initiative, and conscience.

— Dušan Simović

The bulk of the 1st Army were able to cross the Danube and began to prepare defences. By the evening of 10 April, the 1st Army was ordered to withdraw from this line and form a defensive line behind the Sava from Debrc to the confluence with the Vrbas river, for which one or two days would be needed. On the night of 10/11 April, the whole 2nd Army Group continued its withdrawal, but units of the 2nd Army on the left flank of the 1st Army that included significant numbers of Croats began to dissolve due to the fifth column activities of the fascist Ustaše and their sympathisers.[20]

Messerschmitt Bf 110's of Zerstörergeschwader 26 destroyed the air reconnaissance assets of the 1st Army on their airfields over successive days

At dawn on 11 April, Hungarian forces,[21] consisting with the Mobile, IV and V Corps of Vezérezredes (Lieutenant General) Elemér Gorondy-Novák's 3rd Army,[22] crossed the Yugoslav border north of Osijek and near Subotica,[23] overcame Yugoslav border guards and advanced on Subotica and Palić.[24] The XLVI Motorised Corps continued to push east south of the Drava, with the 8th Panzer Division capturing Našice, Osijek on the Drava, and Vukovar on the Danube, followed by the 16th Motorised Infantry Division which advanced east of Našice, despite bridge demolitions and poor roads.[21][19] The 8th Panzer Division had effectively routed the 2nd Army Group by 11 April.[25] On the same day, Messerschmitt Bf 110's of I Group of the 26th Heavy Fighter Wing (German: Zerstörergeschwader 26, ZG 26) destroyed several 1st Air Reconnaissance Group Breguet 19s at Ruma. The rest were flown to Bijeljina, but were destroyed the following day when I/ZG 26 swept over the airfield in one of the most effective attacks of the campaign.[26] On the night of 11/12 April, the 8th Panzer Division captured Sremska Mitrovica on the Sava at 02:30,[19] destroyed a bridge over the Danube at Bogojevo,[27] and advanced on Lazarevac about 32 kilometres (20 mi) south of Belgrade.[19] These advances delayed the withdrawal of the 2nd Army Group south of the Sava.[24]

By 12 April, the withdrawal of the 2nd Army Group was being threatened from the left flank, with 2nd Army having "no combat importance at all". On the right flank, 6th Army attempted to regroup while being pressed by the 11th Panzer Division as it drove towards Belgrade.[27] West of Belgrade, remnants of the 2nd Army Group tried to establish a line along the Sava, but XLVI Motorised Corps had already captured the bridges. When elements of the 8th Panzer Division captured Zemun without a fight, they captured 1st Army's rear area units. On 12 April, the 1st Army's 3rd Cavalry Division counter-attacked at Šabac and pushed the Germans back across the Sava.[24] The Hungarians pursued the 1st Army south, and occupied the area between the Danube and the Tisza meeting virtually no resistance.[23][28] Serb Chetnik irregulars fought isolated engagements,[29] and the Hungarian General Staff considered irregular resistance forces to be their only significant opposition.[30][31] The Hungarian 1st Parachute Battalion captured canal bridges at Vrbas and Srbobran.[29] This, the first airborne operation in Hungarian history, was not without incident. The battalion's aircraft consisted of five Italian-made Savoia-Marchetti SM.75 transport aircraft formerly with the civilian airline MALERT, but pressed into service with the Royal Hungarian Air Force (Hungarian: Magyar Királyi Honvéd Légierő, MKHL) at the start of the European war.[32] Shortly after takeoff from the airport at Veszprém-Jutas on the afternoon of 12 April, the command plane, code E-101, crashed with the loss of 20[33] or 23 lives, including 19 paratroopers. This was the heaviest single loss suffered by the Hungarians during the Yugoslav campaign.[32] Meanwhile, Sombor was captured against determined Chetnik resistance, and Subotica was also captured.[29]

The Germans struggled along poor roads during their drive east towards Belgrade

On the evening of 12 April, elements of the SS Motorised Infantry Division Reich, under command of XLI Motorised Corps crossed the Danube in pneumatic boats and captured Belgrade without resistance. About the same time, most of the elements of XLVI Motorised Corps that were approaching Belgrade from the west were redirected away from the capital. Elements of the 8th Panzer Division did continue their thrust to capture the Sava bridges to the west of Belgrade, and entered the city during the night. The rest of the 8th Panzer Division turned southeast and drove towards Valjevo to link up with the left flank of the First Panzer Group southwest of Belgrade. The 16th Motorised Infantry Division was redirected south across the Sava, and advanced toward Zvornik.[34]

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