The 1st Army Group was a Royal Yugoslav Army formation mobilised prior to the German-led Axis invasion of Yugoslavia in April 1941 during World War II. It consisted of the 4th Army, 7th Army, and the 1st Cavalry Division, which was the army group reserve. It was responsible for the defence of northwestern Yugoslavia, with the 4th Army defending the eastern sector along the Yugoslav–Hungarian border, and the 7th Army defending the western sector along the borders with Germany and Italy. Like all Yugoslav formations at the time, the 1st Army Group had serious deficiencies in both mobility and firepower as well as internal friction among the different ethnic groups, particularly between Serbs and Croats.

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 and precipitate war. When the invasion commenced on 6 April, the component formations of 1st Army Group were only partially mobilised, and on the first day the Germans seized bridges over the Drava River in both sectors and several mountain passes in the 7th Army sector. In the 4th Army sector, the formation and expansion of German bridgeheads across the Drava were facilitated by fifth column elements of the Croat fascist and ultranationalist Ustaše. Mutiny by Croat soldiers broke out in all three divisions of the 4th Army in the first few days, causing significant disruption to mobilisation and deployment. The 1st Army Group was also weakened by fifth column activities within its major units when the chief of staff and chief of operations of the headquarters of 1st Army Group aided both Ustaše and Slovene separatists in the 4th and 7th Army sectors respectively. The revolts within the 4th Army were of great concern to the commander of the 7th Army, Diviziski đeneral Dušan Trifunović, but the army group commander, Armijski đeneral Milorad Petrović, did not permit him to withdraw from border areas until the night of 7/8 April, which was followed by the German capture of Maribor as they continued to expand their bridgeheads.

The 4th Army also began to withdraw southwards on 9 April, and on 10 April it quickly ceased to exist as an operational formation in the face of two determined armoured thrusts by the XXXXVI Motorised Corps, one of which captured Zagreb that evening where a newly formed Croatian government hailed the entry of the Germans. Italian offensive operations also began, with thrusts towards Ljubljana and down the Adriatic coast, capturing over 30,000 Yugoslav troops near Delnice. When fifth column elements arrested the staffs of the 1st Army Group, 4th Army and 7th Army on 11 April, the 1st Army Group effectively ceased to exist. On 12 April, a German armoured column linked up with the Italians near the Adriatic coast, encircling the remnants of the withdrawing 7th Army. Remnants of the 4th Army attempted to establish defensive positions in northeastern Bosnia, but were quickly brushed aside by German armour as it drove towards Sarajevo. The Yugoslav Supreme Command unconditionally surrendered 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 1 December 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 very limited firepower. Generals with mindsets 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

Yugoslav war plans saw the headquarters of the 1st Army Group being raised at the time of mobilisation. It was to be commanded by Milorad Petrović, and was to control the 4th Army, commanded by Petar Nedeljković, the 7th Army, commanded by Dušan Trifunović, and the 1st Cavalry Division. The 4th Army was organised and mobilised on a geographic basis from the peacetime 4th Army District. On mobilisation it would consist of three divisions, a brigade-strength infantry detachment, one horsed cavalry regiment and one infantry regiment, and was supported by artillery, anti-aircraft artillery, border guards, and air reconnaissance elements of the Royal Yugoslav Army Air Force (, VVKJ). The troops of the 4th Army included a high percentage of Croats. The 7th Army did not have a corresponding peacetime army district, and, like the headquarters of the 1st Army Group, was to be formed at the time of mobilisation. It would consist of two divisions, two brigade-strength mountain detachments and a brigade-strength infantry detachment, with field and anti-aircraft artillery support, and also had VVKJ air reconnaissance assets available. The 7th Army included a high proportion of Slovenes, but also some ethnic Germans. The 1st Cavalry Division was a horsed cavalry formation that existed as part of the peacetime army, although significant parts of the peacetime division were earmarked to join other formations when they were mobilised. The 1st Army Group did not control any army group-level support units.

Mobilisation and deployment plan After unrelenting political pressure from Adolf Hitler, Yugoslavia signed the Tripartite Pact on 25 March 1941. On 27 March, a military coup d'état overthrew the government that had signed the pact, and a new government was formed under the commander of the VVKJ, Armijski đeneral Dušan Simović. A general mobilisation was not initiated by the new government until 3 April 1941 not to provoke Germany any further and thus precipitating war. However, on the same day as the coup, Hitler issued Führer Directive 25 that called for Yugoslavia to be treated as a hostile state; on 3 April, Führer Directive 26 was issued, detailing the plan of attack and command structure for the German-led Axis invasion, which was to commence on 6 April.

The deployment plan for 1st Army Group saw the 4th Army deployed in a cordon behind the Drava between Varaždin and Slatina, with formations centred around the towns of Ivanec, Varaždin, Koprivnica and Virovitica. The 7th Army deployment plan saw its formations placed in a cordon along the border region from the Adriatic coast near Senj north to Kranj in the Julian Alps and along the German border to Maribor. It was envisaged that the 1st Cavalry Division would be located in and around Zagreb as the reserve for the 1st Army Group. The Yugoslav historian Velimir Terzić describes the mobilisation of all formations of the 1st Army Group on 6 April as "only partial", and notes that there was a poor response to mobilisation orders for both men and animals. To the right of the 1st Army Group was the 2nd Army of the 2nd Army Group, with the army group boundary running from just east of Slatina through Požega towards Banja Luka. On the left flank of the 1st Army Group, the Adriatic coast was defended by Coastal Defence Command.

Operations during German invasion

The invasion of Yugoslavia, was a German-led attack on the Kingdom of Yugoslavia by the Axis powers which began on 6 April 1941 during World War II. The order for the invasion was put forward in "Führer Directive No. 25", which Adolf Hitler issued on 27 March 1941, following the Yugoslav coup d'état.

The invasion commenced with an overwhelming air attack on Belgrade and facilities of the Royal Yugoslav Air Force (VVKJ) by the Luftwaffe (German Air Force) and attacks by German land forces from southwestern Bulgaria. These attacks were followed by German thrusts from Romania, Hungary and the Ostmark. Italian forces were limited to air and artillery attacks until 11 April, when the Italian army attacked towards Ljubljana (in modern-day Slovenia) and through Istria and Lika and down the Dalmatian coast. On the same day, Hungarian forces entered Yugoslav Bačka and Baranya, but like the Italians they faced practically no resistance. A Yugoslav attack into the northern parts of the Italian protectorate of Albania met with initial success, but was inconsequential due to the collapse of the rest of the Yugoslav forces.

Scholars have proposed several theories for the Royal Yugoslav Army's sudden collapse, including poor training and equipment, generals eager to secure a quick cessation of hostilities, and a sizeable Croatian nationalist fifth column. The invasion ended when an armistice was signed on 17 April 1941, based on the unconditional surrender of the Yugoslav army, which came into effect at noon on 18 April. Yugoslavia was then occupied and partitioned by the Axis powers. Some areas of Yugoslavia were annexed by neighboring Axis countries, some areas remained occupied, and in other areas Axis puppet states such as the Independent State of Croatia (, or NDH) were created during the invasion on 10 April. Along with Italy's stalled invasion of Greece on 28 October 1940, and the German-led invasion of Greece (Operation Marita) and invasion of Crete (Operation Merkur), the invasion of Yugoslavia was part of the German Balkan Campaign ().

6–9 April

4th Army sector

German Army headquarters wanted to capture the bridges over the Drava intact, and from 1 April had issued orders to Generaloberst Maximilian von Weichs's 2nd Army to conduct preliminary operations aimed at seizing the bridge at Barcs and the railway bridge at Zákány by coup de main. As a result, limited objective attacks were launched along the line of the Drava by the XXXXVI Motorised Corps of General der Panzertruppe Heinrich von Vietinghoff, despite the fact that they were not expected to launch offensive operations until 10 April.