It was partly mobilised from the Osječka military district, and, like all Yugoslav infantry divisions of the time, was a very large and unwieldy formation which was almost entirely reliant on animal transport for mobility.

Commanded by Brigadni đeneral Ratko Raketić, it was largely manned by Croat troops, many of whom saw the Germans as potential liberators from Serbian oppression during the interwar period, and the division also lacked modern arms and sufficient ammunition.

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.

It was formed around the nucleus of the victorious Royal Serbian Army, as well as armed formations raised in regions formerly controlled by Austria-Hungary.

[3] 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.

[7][8] 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.

[17] In September 1939, after the German invasion of Poland, the 40th ID underwent a trial mobilisation which included a large proportion of its units.

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

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

[43] Following up the withdrawal of the divisional cavalry battalion, the Germans seized Suhopolje, west of Virovitica, cutting the main road to Slatina,[43] and the rebel Croat troops at Bjelovar made contact with them.

[22] Other reinforcements included elements of the 4th Army anti-aircraft units sent from Lipik, but the divisional artillery regiment had not completed mobilisation.

[44] At 11:15, Nedeljković arrived at divisional headquarters and shortly afterwards ordered Raketić to launch a counterattack on the German bridgehead at Barcs at dawn the following day.

Nedeljković also visited Divizijski đeneral[f] Dragoslav Milosavljević, the commander of the 17th ID on the right flank of the 4th Army, to arrange support from that division during the pending attack.

[43] However, because the majority of that division's troops had yet to arrive from Bosnia, all it was able to do was advance its left flank, stationing battalions in Čačinci and Crnac west of Slatina.

In an indication of the state of his division, during a visit to the front line, Raketić and his chief of staff were fired at by troops of the 42nd Infantry Regiment.

[51] The second would see Generalmajor Walter Neumann-Silkow's 8th Panzer Division break out of the Barcs bridgehead and turn east between the Drava and Sava rivers to attack towards Belgrade.

Having abandoned the counterattack, Raketić decided to establish a defensive line at Pćelić to hinder German movement east towards Slatina.

[54] The 8th Panzer Division continued southeast between the Drava and Sava rivers, and meeting almost no further resistance, had reached Slatina by evening.

[52] Right flank elements of the 8th Panzer Division penetrated south into the Bilogora range, reaching Daruvar and Voćin by evening.

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:[52]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.The XXXXVI Motorised Corps encountered little resistance from the 40th ID, and by the evening of 10 April the whole 4th Army was disintegrating.

At midnight, 2nd Army headquarters declared that the Yugoslav northern front had been decisively defeated, and tasked corps engineer units to consolidate bridging across the major rivers.

[57] 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.

[58] 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.

A fragment of the 40th ID, numbering around 300 troops, which had been holding a position on the Sava at Bosanska Gradiška, retreated to Jajce via Banja Luka.

When they arrived at Jajce, Nedeljković ordered them to take up blocking positions in the narrow Vrbas valley at Krupa on the road between Banja Luka and Mrkonjić Grad.

The cadet battalion at Ključ managed to briefly delay the German advance through Mrkonjić Grad, but were overcome by tanks and air attacks.

[61] Early on 15 April, the northern column of the 14th Panzer Division closed on Doboj, and after overcoming resistance around that town, arrived in Sarajevo at 20:45.

Before noon, Nedeljković received orders that a ceasefire had been agreed, and that all 4th Army troops were to remain in place and not fire on German personnel.

[63] Yugoslavia was then occupied and dismembered by the Axis powers, with Germany, Italy, Hungary, Bulgaria and Albania all annexing parts of its territory.