The 'Drava Defensive Operation' was a Bulgarian defensive undertaking, also known as the 'Battle of the Transdanubian Hills', by General Polkovnik Vladimir D. Stoychev’s Bulgarian 1^st Army during the German 'Frühlingserwachen' (ii) offensive in southern Hungary (6/21 March 1945).

The German attacks failed to achieve their objectives and were then undone by the first stages of the Soviet 'Vienna Strategic Offensive Operation' (13 March/15 April).

The German forces to the south of Lake Balaton comprised two groupings. The northern group was General Maximilian de Angelis’s 2^nd Panzerarmee, which had the town of Kaposvár as its objective. Facing the 2^nd Panzerarmee's northern flank was General Leytenant Mikhail N. Sharokhin’s Soviet 57^th Army, while the area in front of the 2^nd Panzerarmee's southern flank was defended by the III Corps of Stoychev’s Bulgarian 1^st Army.

The Germans' southern group was General Werner von Erdmannsdorff’s LXXXXI Corps of Generaloberst Alexander Löhr’s Heeresgruppe 'E' in northern Yugoslavia. The LXXXXI Corps was concentrated near Donji Miholjac and Valpovo, and on the other side of the Drava river, at Donji Miholjac, the Bulgarian IV Corps defended the routes to Pécs and Mohács. To the north-east of Valpovo and across the Drava river, General Kosta Nađ's 3^rd Army of Marshal Josip Broz Tito’s Yugoslav National Liberation Army defended the routes into north-eastern Yugoslavia. At this time the main task of the LXXXXI Corps was 'Waldteufel' with the the town of Mohács as its primary objective.

In tandem with other German attacks that formed part of 'Frühlingserwachen' (ii), the German advance to the south of Lake Balaton was directed to the encirclement and subsequent destruction of the Soviet 57^th Army and Bulgarian 1^st Army to the west of the Danube river. The 57^th Army was the most southerly Soviet formation on the Eastern Front, and in the area to the south of that in which the 57^th Army was operating, the Germans of the 2^nd Panzerarmee and Heeresgruppe 'E' faced Soviet allies in the form of the Bulgarian forces in southern Hungary and the Yugoslav forces in Yugoslavia. The Germans believed that an offensive in these areas offered the likelihood of falling on less well-equipped Bulgarian and Yugoslav forces, thereby opening the way to the possibility of rolling up part of the Soviets' southern flank. The Soviets also appreciated this fact, and the region was therefore of particular concern to Marshal Sovetskogo Soyuza Fyedor I. Tolbukhin, whose 3^rd Ukrainian Front was deployed in southern Hungary and incorporated both the 57^th Army and the Bulgarian 1^st Army. (The formation to the south of the Bulgarians was the Yugoslav 3^rd Army, which was not subordinated to 3^rd Ukrainian Front.) Alerted that a German offensive to the south of Lake Balaton was imminent, Tolbukhin deployed additional artillery units to reinforce the 57^th Army and the Bulgarian III Corps.

General Rudolf Konrad’s LXVIII Corps of the 2^nd Panzerarmee, with Generalmajor Eberhard von Schuckmann’s 71^st Division and SS-Brigadeführer und Generalmajor der Waffen-SS Desiderius Hampel’s 13^th Waffen-Gebirgsdivision der SS 'Handschar' (kroatisch Nr 1), made the main thrust in the area to the south of Lake Balaton on March 6 with an attack on the 57^th Army and the 10^th Division of the Bulgarian III Corps. With three infantry divisions (the 16^th Division was redeployed to the assistance of the IV Corps), and aided by a thaw which turned the ground into a muddy morass, the Bulgarians held, as too did the 57^th Army. Attacking on frontage of 6.25 miles (10 km), the LXVIII Corps, which was also designated as the Gruppe 'Konrad' for this operation, could not break through the Soviet and Bulgarian defences and had to content itself with pinning down the Soviet and Bulgarian forces in this region. By 12 March the LXVIII Corps had shifted the axis of its attack to the 57^th Army, and then managed to penetrate some 6.25 miles (10 km) into the Soviet defensive before becoming bogged down.

In the Donji Miholjac area, the LXXXXI Corps attacked on March 6 with about 35,000 men, and Generalleutnant Hartwig von Ludiger’s 104^th Jägerdivision and Generalleutnant Albrecht Baier’s 297^th Division crossed the Drava river and, pressing the Bulgarian IV Corps' widely deployed 3^rd Division, occupied an area almost 12.5 miles (20 km) deep and 6.25 miles (10 km) wide.

On 7 March Stoychev ordered the Bulgarian III Corps' 16^th Division to reinforce the 3^rd Division at Donji Miholjac, but this required a march of more than 50 miles (80 km). During 8/9 March, some Bulgarian elements began to withdraw from Pécs, and in response Tolbukhin ordered General Major Pavel A. Artyushenko’s Soviet CXXXIII Corps out of reserve to reinforce the Bulgarian IV Corps. In the course of their march, the Soviet troops encountered Bulgarian troops and engaged them as a result of the fact that the Bulgarian helmets resembled those of the Germans. Reinforced by Soviet artillery units and supported by ground-attack aircraft, the CXXXIII Corps then took up position at the critical junction of the Bulgarian and Yugoslav forces. Ironically, in this part of the campaign the Germans found themselves facing PzKpfw IV battle tanks, of German design and manufacture, operated by the Bulgarian forces.

Between 12 and 19 March, after fierce fighting in the villages of Drávaszabolcs, Drávapalkonya and Drávacsehi, and with the support of the CXXXIII Corps, the Soviet 84^th Division from the 3^rd Ukrainian Front reserve and the Bulgarian 16^th Division, the Bulgarian formation halted the German advance. Now appreciating that the 2^nd Panzerarmee could not break through and was in fact coming under increasing pressure from the Soviets and Bulgarians, the Germans evacuated the bridgehead across the Drava river at Donji Miholjac between 18 and 20 March.

Near Valpovo, Generalmajor Gerhard Henke’s 11^th Felddivision (L), supported by two regiments of dismounted Cossacks cavalrymen, had crossed the Drava river and driven about 1.25 miles (2 km) to the north-east by March 8. In the face of resistance by General-major Danilo Lekić-Spanac’s Yugoslav XII Corps, this attack then slowed and by 21 March the Yugoslav 3^rd Army had pushed the Germans away from Valpovo.

The success of the Bulgarian, Soviet and Yugoslav forces during the combat in the Transdanubian Hills caused the failure of the planned German advance in the region and helped set the scene for the 'Vienna Strategic Offensive Operation'. Moreover, the involvement of the LXXXXI Corps of Heeresgruppe 'E' had required redeployment which significantly weakened the overall German defence in Yugoslavia, and this was a situation rapidly exploited by the Yugoslav National Liberation Army.

The overall conduct of 'Frühlingserwachen' (ii) has also been criticised because of the German failure to deploy the stronger formations and units of SS-Oberstgruppenführer und Generaloberst der Waffen-SS Josef Dietrich’s 6^th SS Panzerarmee against the relatively weak Soviet and Bulgarian defences in the area to the south of Lake Balaton. The 6^th SS Panzerarmee was instead deployed in the area to the north of Lake Balaton, which the Hungarian military had already warned was unsuitable for armoured warfare.