The formation and expansion of German bridgeheads were facilitated by fifth-column elements of the Croatian fascist Ustaše organisation and their sympathisers among the Croat-majority populace of the 4th Army sector.

Under the leadership of its commander, Armijski đeneral[b] Petar Nedeljković, the mostly ethnic Serb remnants of the 4th Army attempted to establish defensive positions in northeastern Bosnia, but were brushed aside by the 14th Panzer Division as it drove east towards Sarajevo, which fell on 15 April.

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.

[5] 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.

[9][10] 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.

[34] Similar operations occurred on the extreme left flank of the 4th Army, where raiding parties and patrols from LI Infantry Corps, commanded by General der Infanterie[f] Hans-Wolfgang Reinhard, seized high ground on the south side of the Drava.

The local attacks were sufficient to inflame dissent within the largely Croat 4th Army, who refused to resist Germans they considered their liberators from Serbian oppression during the interwar period.

[38] In the afternoon, German Junkers Ju 87 Stuka dive bombers of Sturzkampfgeschwader 77 escorted by Messerschmitt Bf 109E fighters caught the Breguet 19s of the 4th Air Reconnaissance Group on the ground at Velika Gorica, destroying most of them.

These attempts were only partially successful, due to the influence of Ustaše propaganda and the countermanding of the demolition orders by the chief of staff of the 27th Infantry Division Savska, Major Anton Marković.

[41] About 07:30, the commander of the Yugoslav 1st Army Group, Armijski đeneral Milorad Petrović, met with Nedeljković at Zagreb and ordered him to go to Koprivnica and prepare a counterattack against the bridgehead, to commence at 15:00.

[36] By late evening on 7 April, Petrović's reports to Supreme Headquarters noted that the 4th Army was exhausted and its morale had been degraded significantly, and that Nedeljković concurred with his commander's assessment.

[42] On 8 April, the German XXXXVI Motorised Corps continued with its limited objective attacks to expand their bridgeheads at Barcs and Zákány, capturing Kotoriba, a village upstream from Legrad.

On the same day, Maček, who had returned to Zagreb after briefly joining Simović's post-coup government, agreed to send an emissary to the 108th Infantry Regiment of the 40th ID urging them to obey their officers, but they did not respond to his appeal.

[50] By the evening of 8 April, the Yugoslav Supreme Headquarters was under the mistaken impression that the situation in the 4th Army's area of operation was relatively good, believing the penetration of German troops had been temporarily halted.

Marić would not inform 4th Army headquarters of this idea, so Georgijević went to Zagreb to speak to Petrović, and to further urge him to withdraw all troops that still wanted to fight to a line south of the Sava.

According to Georgijević, Petrović ordered him to tell Marić to consider disarming his Croat troops, and to continue to hold positions on the line of the Bilogora, but to conduct a fighting withdrawal towards Zagreb and Sisak if the German pressure was too great.

[52] The rebels in Bjelovar used the telegraph station and telephone exchange in the town to issue false orders to parts of the 104th Infantry Regiment directing them to withdraw from their positions.

[55] Following up the withdrawal of the divisional cavalry squadron, the Germans seized Suho Polje, west of Virovitica, cutting the main road to Slatina,[55] and the rebel Croat troops at Bjelovar made contact with them.

[52] On the evening of 9 April, Generaloberst[j] Maximilian von Weichs, commander of the German 2nd Army, was ready to launch major offensive operations from the bridgeheads on the following day.

[17] Early on 10 April, Pukovnik Franjo Nikolić, the head of the operations staff with the headquarters of the 1st Army Group,[60] left his post and visited the senior Ustaše leader Slavko Kvaternik in Zagreb.

[62] On the same day, the 14th Panzer Division, supported by dive bombers, crossed the Drava and drove southwest towards Zagreb on snow-covered roads in extremely cold conditions.

Nedeljković replied that he no longer had an army, and suggested that all Serb officers and men be ordered back to form a defensive line along the Sava and Una rivers.

[68] Late in the 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:[17] All troops must engage the enemy wherever encountered and with every means at their disposal.

At midnight, 2nd Army headquarters declared that the Yugoslav northern front had been decisively defeated, and tasked corps engineer units to consolidate bridging across major rivers, particularly over the Sava at Brežice west of Zagreb to facilitate the advance of the 14th Panzer Division towards Karlovac.

[69] Held up by freezing weather and snow storms, LI Corps was approaching Zagreb from the north,[66] and broke through a hastily established defensive line between Pregrada and Krapina.

The German-installed NDH government called on all Croats to stop fighting, and in the evening, LI Infantry Corps entered Zagreb and relieved the 14th Panzer Division.

[70] The break-up of the 4th Army and westward thrust of the 14th Panzer Division opened up the Bosnian interior to the Germans, and also threatened the left flank of the Yugoslav forces attempting to establish a defensive line along the Sava.

[71] The German orders for the following day were to pursue the remnants of the Yugoslav Army through Bosnia towards Sarajevo, where they would be met by the First Panzer Group attacking from the south and east.

[74] About 18:00, Nedeljković received a telephone call from Simović and reported that Bosanska Dubica, Bosanski Novi and Prijedor had all fallen, but that the bridges on the Sava and Una would be demolished later that night, and that he and his remaining staff would be leaving at 20:00 for Jajce.

[74] The following day, the northern column of the 14th Panzer Division drove via Glina and crossed the Una at both Bosanska Kostajnica and Bosanski Novi before continuing its push east.