Despite concerns over a possible Axis invasion, orders for the general mobilisation of the Royal Yugoslav Army were not issued by the government until 3 April 1941, not to provoke Germany any further following the military coup d'état and precipitate war.

When the invasion commenced on 6 April, the 7th Army was only partially mobilised, and on the first day the Germans seized several mountain passes and bridges over the Drava river.

Italian offensive operations began the following day, with thrusts towards Ljubljana and down the Adriatic coast that resulted in the capture of more than 30,000 Yugoslav troops near Delnice.

When fifth column supporters of the Croatian nationalist Ustaše movement arrested the headquarters staff of the 7th Army later that day, the formation effectively ceased to exist.

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.

[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.

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.

[11] Zagreb, Karlovac, Trebnje and Velike Lašče were key centres for the mobilisation and concentration of the 7th Army due to their good rail infrastructure.

[20] Border guard units were to man fortifications along the Italian and German frontiers in the 7th Army area of responsibility, and consisted of:[22] After unrelenting pressure from Adolf Hitler, Yugoslavia signed the Tripartite Pact on 25 March 1941.

[29] On the morning of 6 April, German aircraft conducted surprise attacks on Yugoslav airfields in the 7th Army area, including Ljubljana and Cerklje, where the 6th Air Reconnaissance Group was based.

[34] By the end of the first day, LI Infantry Corps had occupied Gornja Radgona, Murska Sobota and Radenci, and had crossed the Drava near Sveti Duh.

[29] Later that day, German Junkers Ju 87 Stuka dive bombers of Sturzkampfgeschwader 77, escorted by Messerschmitt Bf 109E fighters, caught the Breguet 19s of the 6th Air Reconnaissance Group on the ground at Cerklje and Brege, destroying most of them.

German and Italian air attacks interfered with the deployment of troops and command was hampered by reliance on civilian telegraph and telephone services.

[33] In the early hours of 7 April, three Blenheims of the Yugoslav 8th Bomber Regiment took off from Rovine to bomb the railway junction at Feldbach, but became disoriented in bad weather.

Only one aircraft found a target in Austria, bombing a bridge and road near Steyr before continuing on to Wiener Neustadt where it was hit by anti-aircraft fire and made an emergency landing.

The German thrusts towards Ptuj and Maribor broke through the Yugoslav defensive line Pesnica–Lenart–Sveta Trojica v Slovenskih Goricah–Kapelski Vrh, but those advancing towards Dravograd were held up by the 6th Border Regiment and a battalion of the 38th Infantry Division Dravska.

[42] Disregarding orders from above, Palten exploited their withdrawal by leading his kampfgruppe south towards the town, and crossing the Pesnica river in inflatable boats, leaving his unit vehicles behind.

[46] In view of German success, the Italian 2nd Army in north-eastern Italy accelerated its preparations and issued orders for its V and XI Corps to conduct preliminary operations aimed at improving their starting positions for the planned attack on Yugoslavia.

Rupnik and the head of the operations staff of the headquarters of the 1st Army Group, Pukovnik Franjo Nikolić, hid the orders from Petrović and did not carry them out.

[50] Before it arrived, the Ustaše, supported by German agents, had proclaimed the creation of the Independent State of Croatia (Croatian: Nezavisna Država Hrvatska, NDH).

[49] This crossing point was a partially destroyed bridge, guarded by a single platoon of the 1st Bicycle Battalion of Detachment Ormozki, the far left formation of the 4th Army.

This crossing, combined with the withdrawal of the 38th Infantry Division Dravska from the line from Slovenska Bistrica to Ptuj, exposed the left flank of Detachment Ormozki.

[54] Luftwaffe reconnaissance sorties revealed that the main body of the 7th Army was withdrawing towards Zagreb, leaving behind light forces to maintain contact with the German bridgeheads.

[34] On 10 April, as the situation was becoming increasingly desperate throughout the country, Simović, who was both the Prime Minister and Chief of the General Staff, broadcast the following message:[55] All troops must engage the enemy wherever encountered and with every means at their disposal.

[57] Chaos ensued throughout the 7th Army, whose Croat and Slovene soldiers could hear fifth column radio broadcasts telling them of their pending encirclement by the Germans and encouraging them to return to their homes and not fight against the invaders.

[60] Held up by freezing weather and snow storms, LI Infantry Corps was approaching Zagreb from the north,[52] and broke through a hastily established defensive line between Pregrada and Krapina.

[60] To assist the Italian advance, the Luftwaffe attacked Yugoslav troops in the Ljubljana region, and the 14th Panzer Division, which had captured Zagreb on 10 April, drove west to encircle the withdrawing 7th Army.

[52] When the Italian 14th Infantry Division Isonzo entered Ljubljana, a delegate of the NszS greeted its commander, Generale di Divisione[j] Federico Romero, and symbolically handed him the keys to the city.

[64] Remnants of the 4th Army conducted a fighting withdrawal through Bosnia towards Sarajevo over the following days, pursued by the 14th Panzer Division and elements of LI Infantry Corps but a ceasefire was declared at noon on 15 April.