[1] 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.

The army budget remained tight, and as tensions rose across Europe during the 1930s, it became difficult to secure weapons and munitions from other countries.

[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.

[21] The 5th Army was commanded by Armijski đeneral[c] Vladimir Cukavac, his chief of staff was Pukovnik[d] Fran Tomše.

[30] The planned deployment of the 5th Army from north to south was:[31] At 22:00 on 5 April, German Detachment Đerdap began crossing of the Danube at the Yugoslav–Romanian border in two groups.

[35] At 5:00 on 6 April, 20 Luftwaffe Messerschmitt Bf 109Es attacked Sorlince airport, destroying all aircraft of the 2nd Air Reconnaissance Group attached to the 5th Army.

(Jagd)/LG 2 Herbert Ihlefeld was hit by small arms fire forcing him to crash-land near the village of Donji Dušnik, after which he was imprisoned by the Yugoslav Army.

German documents remark that Yugoslav units conducted an attack 3 kilometers northwest of the village of Miloslavtsi in Bulgaria.

Commander of 34th ID Toplička Vladislav Kostić ordered his units to open artillery fire on 11th Panzer Division after he was informed of its attack.

After the fall of Caribrod, the 61st Artillery Regiment opened fire on the tanks, while it didn't manage to do much damage, it slowed down German advance.

Fall of Bela Palanka made the Staff of the 5th Army in danger so it moved from Niška Banja to Kruševac during the night of 8–9 April.

[49] Route from Niš towards Prokuplje was also defended by the elements of the 2nd Cavalry Division which formed Detachment Mramorski stationed in the village of Mramor.

[51] This detachment and two divisions had already begun retreating during the previous night, while they were troubled by harsh weather, the units weren't attacked.

After finding about its further retreat, the Yugoslav Supreme Command ordered Armijski đeneral Cukavac to return to Kraljevo with his staff.

North of Paraćin, at Ćuprija, the division fought against the 8th Cavalry Divizion of the 8th ID Krajinska, which retreated after destroying one tank.

This resistance was defeated and Germans continued their advance towards Jagodina, where citizens had already placed stone barricades and were armed by the vice president of the municipality.

They were slowed down at the village of Bunar by two artillery batteries of 9th ID Timočka and a firefighter company which blockaded the road with two trucks, after a half an hour long battle Yugoslav units retreated.

Germans continued their advance towards Kragujevac after this battle but they were stopped by heavy artillery fire from the city at the village of Gornja Sabanta, where they spent the night.

[57] In the morning of 10 April, weaker elements of the 11th Panzer Division attacked two infantry battalions of the Detachment Vlasinski and the 50th ID Drinska stationed in Leskovac.

[55] After receiving reinforcements during the night, German forces stationed at Gornja Sabanta mounted an assault towards Kragujevac in the morning of 11 April.

[61] In the afternoon, one detachment of the 11th Panzer Division attacked one Yugoslav battalion and a battery at the village of Bovan, near Soko Banja.

Yugoslav forces were victorious and completely destroyed the detachment, capturing 25 soldiers, two tanks, two anti-aircraft guns, and six motorcycles.

[65] At around 7:00 on 12 April, German armored and infantry units attacked Detachment Blacki of the 2nd Cavalry Division stationed at Blace.

Battle there lasted until twilight, when German artillery opened fire on Yugoslav units from all sides, forcing them to retreat.

Around 3,500 unarmed soldiers from 50th ID Drinska managed to escape encirclement and group up between Brus and Aleksandrovac, from where they were directed to an area west of Kraljevo.

An hour later, commander of 2nd Cavalry Division Dimitrije Predić ordered formation of Detachment Razbojni from the units stationed at Razbojna.

[70] Detachment Gornjomilanovački, part of the 1st ID Cerska, engaged in a battle with German motorized and armored column on 13 April.