The 4th Army was a Royal Yugoslav Army formation mobilised prior to the German-led Axis invasion of the Kingdom of Yugoslavia during World War II. It was drawn from the peacetime 4th Army District. When mobilised, it consisted of three divisions, a brigade-strength detachment, one horse cavalry regiment and one independent infantry regiment. It formed part of the 1st Army Group, and was responsible for defending a large section of the Yugoslav–Hungarian border, being deployed behind the Drava river between Varaždin and Slatina. Like all Yugoslav formations at the time, the 4th Army had serious deficiencies in both mobility and firepower.

Despite concerns over a possible Axis invasion, orders for the general mobilisation of the Royal Yugoslav Army were not issued by the government until 3April 1941, in order to not provoke Germany any further following the military coup d'état and precipitate war. When the invasion commenced on 6April, the 4th Army was only partially mobilised, and this weakness was exacerbated by fifth-column activities within its major units and higher headquarters. Revolts of Croat soldiers broke out in all three divisions in the first few days, causing significant disruption to their mobilisation and deployment. The town of Bjelovar was taken over by rebel troops. Widespread desertions of Croat troops, many of whom turned on their Serb comrades, made control even more difficult. German activity in the 4th Army sector in the first four days included limited objective attacks to seize crossings over the Mura and Drava rivers, along with air attacks by the Luftwaffe.

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. Elements of the 4th Army did put up scattered resistance to the Germans, but it began to withdraw southwards on 9April, and on 10 April it quickly ceased to exist as an operational formation in the face of two determined armoured thrusts by XXXXVI Motorised Corps from bridgeheads at Zákány and Barcs. The 14th Panzer Division captured Zagreb late that day, and the Germans facilitated the proclamation of an independent Croatian state. A senior staff officer at the headquarters of the 1st Army Group who sympathised with the Ustaše issued orders redirecting formations and units of the 4th Army away from the advancing Germans, and fifth-column elements arrested some 4th Army headquarters staff.

Under the leadership of its commander, Armijski đeneral 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. A ceasefire was agreed on that day, and the remains of the 4th Army were ordered to stop fighting. The Yugoslav Supreme Command surrendered unconditionally effective on 18 April.

Background

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 1December 1918, in the immediate aftermath of World War I. The Army of the Kingdom of Serbs, Croats and Slovenes was established to defend the new state. 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. Many former Austro-Hungarian officers and soldiers became members of the new army. From the beginning, much like other aspects of public life in the new kingdom, the army was dominated by ethnic Serbs, who saw it as a means by which to secure Serb political hegemony.

The army's development was hampered by the kingdom's poor economy, and this continued during the 1920s. In 1929, King Alexander changed the name of the country to the Kingdom of Yugoslavia, at which time the army was renamed the Royal Yugoslav Army (, VKJ). 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. 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. Infantry divisions had a wartime strength of 26,000–27,000 men, as compared to contemporary British infantry divisions of half that strength. These characteristics resulted in slow, unwieldy formations, and the inadequate supply of arms and munitions meant that even the very large Yugoslav formations had low firepower. Generals better suited to the trench warfare of World War I were combined with an army that was neither equipped nor trained to resist the fast-moving combined arms approach used by the Germans in their invasions of Poland and France.

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. Attempts to address the disunity came too late to ensure that the VKJ was a cohesive force. 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.

Formation and composition

Peacetime organisation

Yugoslav war plans saw the 4th Army organized and mobilised on a geographic basis from the peacetime 4th Army District, which was divided into three divisional districts, each of which was subdivided into regimental regions. Zagreb, Dugo Selo and Sisak were key centres for the mobilisation and concentration of the 4th Army due to their good rail infrastructure. Prior to the issue of mobilisation orders for the 4th Army, the 4th Army District headquarters had been involved in planning border defences and conducting exercises for border troops, including demolition plans for bridges and other infrastructure in the event of war.

On 8June 1940, the Yugoslav Supreme Command issued orders to the 4th Army District headquarters to make all necessary preparations for defence and demolitions and ordered a 14-day exercise for border troops. 4th Army District headquarters submitted a progress report on this work on 30 January 1941. This report indicated that along the Hungarian border, bunkers and trenches had been constructed for the immediate defence of the Drava river along the Yugoslav-Hungarian border, in particular at Varaždin, Koprivnica, Virovitica and Slatina, but no obstacles such as barbed wire entanglements or anti-tank ditches had been developed.

Wartime organisation

The 4th Army was commanded by Armijski đeneral Petar Nedeljković, and his chief of staff was Brigadni đeneral Anton Lokar. The 4th Army consisted of: 27th Infantry Division Savska 40th Infantry Division Slavonska 42nd Infantry Division Murska Detachment Ormozki (brigade-strength) 127th Infantry Regiment 81st Cavalry Regiment

Army-level support was provided by the motorised 1st Heavy Artillery Regiment, the 81st Army Artillery Regiment, the motorised 4th Anti-Aircraft Battalion, six border guard battalions and the motorised 4th Army Anti-Aircraft Company. The 4th Air Reconnaissance Group comprising eighteen Breguet 19s was attached from the Royal Yugoslav Army Air Force (, VVKJ) and was based at Pleso airfield near Velika Gorica just south of Zagreb. The troops of the 4th Army included a high percentage of Croats.

Deployment plan

The 4th Army was part of the 1st Army Group, which was responsible for the defence of northwestern Yugoslavia. The 4th Army defended the eastern sector along the Hungarian border, and the 7th Army along the German and Italian borders. The 1st Cavalry Division was to be held as the 1st Army Group reserve around Zagreb. On the left of the 4th Army, the boundary with the 7th Army ran from Gornja Radgona on the Mura river through Krapina and Karlovac to Otočac. On the right of the 4th Army was the 2nd Army of the 2nd Army Group, the boundary running from just east of Slatina through Požega towards Banja Luka. The Yugoslav defence plan saw the 4th Army deployed in a cordon behind the Drava between Varaždin and Slatina. The planned deployment of the 4th Army from west to east was: Detachment Ormozki, responsible for the border between Gornja Radgona and the triple border with Germany and Hungary, but with its main defences along the Drava between the confluence with the Dravinja river and the village of Petrijanec, and its headquarters in Klenovnik; 42nd Infantry Division Murska (42nd ID), opposite the Hungarian city of Nagykanizsa, between the triple border with Germany and Hungary and the confluence of the Mura at Legrad, with divisional headquarters at Seketin, just south of Varaždin; 27th Infantry Division Savska (27th ID), opposite the Hungarian village of Gyékényes, between the confluence of the Mura at Legrad and Kloštar Podravski, with divisional headquarters at Kapela, north of Bjelovar; and 40th Infantry Division Slavonska (40th ID), opposite the Hungarian town of Barcs, between Kloštar Podravski and Čađavica, with the main line of defence along the northern slopes of the Bilogora mountain range, and divisional headquarters at Pivnica Slavonska.

Border guard units in the 4th Army area of responsibility consisted of: the 601st Independent Battalion in the sector of the Detachment Ormozki, the 341st Reserve Regiment in the sector of the 42nd ID, the 3rd Battalion of the 393rd Reserve Regiment and 576th Independent Battalion in the sector of the 27th ID, and the 2nd Battalion of the 393rd Reserve Regiment in the sector of the 40th ID.

Mobilisation After unrelenting pressure from Adolf Hitler, Yugoslavia signed the Tripartite Pact on 25 March 1941. Two days later, a military coup d'état overthrew the government that had signed the pact, and a new government was formed under the VVKJ commander, Armijski đeneral Dušan Simović. A general mobilisation was not called by the new government until 3April 1941, out of fear of offending Hitler and thus precipitating war. The same day as the coup, Hitler issued Führer Directive 25, which called for Yugoslavia to be treated as a hostile state and, on 3April, Führer Directive 26 was issued, detailing the plan of attack and command structure for the invasion.

The Yugoslav historian Velimir Terzić described the mobilisation of the 4th Army as a whole on 6April as "only partial", and states the headquarters of the 4th Army was mobilising northeast of Dugo Selo, east of Zagreb, with 30–35 percent of the staff officers and ten percent of the soldiers having reported for duty.

Detachment Ormozki Detachment Ormozki was an ad hoc formation based on the headquarters of the 1st Cavalry Brigade, with an infantry regiment attached from the 32nd Infantry Division Triglavski, and two cavalry regiments and a squadron of cavalry artillery attached from the 1st Cavalry Division. On 6April, it was concentrating in the Ormož area, as follows: the detachment commander and his headquarters staff were in Čakovec; the 39th Infantry Regiment was marching from Celje via Lepoglava to Varaždin, but on 6April had only reached Rogatec; the 6th Cavalry Regiment was mobilising in Zagreb; the 8th Cavalry Regiment was mobilising in Čakovec; a squadron of cavalry artillery was moving from Varaždin to the concentration area, and had reached Vratno; and the 1st Bicycle Battalion had departed Ljubljana, and on 5April had reached Žalec.

The 39th Infantry Regiment was later transferred to the 42nd ID, leaving Detachment Ormozki predominantly as a cavalry formation.

42nd Infantry Division Murska