The 2nd Army Group was not directly attacked during the first few days of the invasion, but events to the east and west of its deployment area resulted in successive orders to withdraw to the lines of the Drava and Danube then the Sava.

Remnants continued to fight along the line of the Sava until early on 14 April, but mass surrenders began that day with tens of thousands of Yugoslav soldiers being taken into captivity by the Germans during their drive on Sarajevo in the centre of the country.

The Kingdom of Serbs, Croats and Slovenes was created with the merger of Serbia, Montenegro and the South Slav-inhabited areas of Austria-Hungary on 1 December 1918, in the immediate aftermath of World War I.

[4] Consequently, at the time World War II broke out in September 1939, the VKJ had several serious weaknesses, which included reliance on draught animals for transport, and the large size of its formations.

[8][9] The weaknesses of the VKJ in strategy, structure, equipment, mobility and supply were exacerbated by serious ethnic disunity within Yugoslavia, resulting from two decades of Serb hegemony and the attendant lack of political legitimacy achieved by the central government.

Fifth column activity was also a serious concern, not only from the Croatian fascist Ustaše and the ethnic German minorities but also potentially from the pro-Bulgarian Macedonians and the Albanian population of Kosovo.

[13] On mobilisation it would consist of one infantry division, one horsed cavalry division, and two brigade-strength infantry detachments, and was supported by artillery, anti-aircraft artillery, and air reconnaissance elements of the Royal Yugoslav Army Air Force (Serbo-Croatian Latin: Vazduhoplovstvo vojske Kraljevine Jugoslavije, VVKJ).

[16] The following day, the situation deteriorated significantly when the German XXXXI Motorised Corps of General der Panzertruppe[c] Georg-Hans Reinhardt 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.

Generalmajor[d] Walter Neumann-Silkow's 8th Panzer Division turned southeast between the Drava and Sava rivers, and meeting almost no resistance and with strong air support, reached the left flank of the 2nd Army at Slatina by evening, despite poor roads and bad weather.

[26] Later that day, as the situation was becoming increasingly desperate throughout the country, Simović, who was both the Prime Minister and Yugoslav Chief of the General Staff, broadcast the following message:[26] All troops must engage the enemy wherever encountered and with every means at their disposal.

By the evening of 10 April, the 2nd Army Group was ordered to withdraw from these positions 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.

[31] The XXXXVI 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 Generalmajor Sigfrid Henrici's 16th Motorised Infantry Division which advanced east of Našice, despite bridge demolitions and poor roads.

The rest were flown to Bijeljina, but all of the air reconnaissance assets of the 2nd Army Group were destroyed the following day when I/ZG 26 swept over the airfield in one of the most effective attacks of the campaign.

[33] On the night of 11/12 April, the 8th Panzer Division captured Sremska Mitrovica on the Sava at 02:30,[26] destroyed a bridge over the Danube at Bogojevo,[34] and advanced on Lazarevac about 32 kilometres (20 mi) south of Belgrade.

[34] West of Belgrade, remnants of the 2nd Army Group tried to establish a line along the Sava, but XXXXVI Motorised Corps had already captured the bridges.

[39] 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[40] or 23 lives, including 19 paratroopers.

[43] On 14 and 15 April, tens of thousands of Yugoslav soldiers were taken prisoner by the Germans during their drive on Sarajevo in the centre of the country, including 30,000 around Zvornik and 6,000 around Doboj.