'Rübezahl I' was a German, Bulgarian and Croat operation against the partisan forces of Marshal Josip Broz Tito in the southern part of German-occupied Yugoslavia (3/26 August 1944).

The increasing concentration of partisan forces in Montenegro made it necessary for the Germans to plan a large-scale operation, but before this initial iteration of 'Rübezahl' could be launched, the movement of the partisan forces toward the Macedonia region required immediate and effective action. The objective of the undertaking was to seal off elements of the partisan main force (I Proletarian, II Assault and XII Assault Corps) on the plateau between the Piva and Tara rivers in Montenegro, and so prevent their movement into the south-western part of Serbia, and to cut off and force the III Assault Corps and elements of the XII Assault Corps to move to the west away from the Drina river in Bosnia and thus prevent their entry into Serbia and a junction with the main force units.

The primary partisan formations involved, totalling about 20,000 men, were the 1^st Proletarian and 6^th Divisions of the I Proletarian Corps, the 3^rd, 29^th and 37^th Divisions of the II Assault Corps, the 16^th and 36^th Divisions of the XII Assault Corps, and the 27^th and 38^th Divisions of the III Assault Corps. The Axis force, totalling about 45,000 men, included as the German component Generalleutnant Walter Stettner Ritter von Grabenhofen’s 1^st Gebirgsdivision, SS-Brigadeführer und Generalmajor der Waffen-SS Karl Reichsritter von Oberkamp’s 7^th SS Gebirgsdivision 'Prinz Eugen', elements of SS-Brigadeführer und Generalmajor der Waffen-SS Desiderius Hampel’s 13^th SS Waffen-Gebirgsdivision 'Handschar' (kroatische Nr 1), elements of Generalleutnant Hermann Fischer’s 181^st Division, elements of Generalleutnant Fritz Neidholdt’s 369^th Division (kroatische), 2^nd Regiment 'Brandenburg' of Generalleutnant Friedrich Kühlwein’s Division 'Brandenburg', elements of the 5^th SS-Polizeiregiment, elements of SS-Hauptsturmführer Karl von Sampler’s Muslimanischen Legion 'Krempler', and the 1^st and 3^rd Regiments of the Montenegrinisches Freiwilligen Korps; as the Croat component two brigades of Četnik auxiliaries as well as 2,000 more operating under the 181^st Division, Ustase Dopukovnik Delko Bogdanić's 1^st Ustase Brigade, Ustase Dopukovnik Kresimir Kurajass 11^th Ustase Brigade, Pukovnik Juraj Čordasić's 8^th Garrison Brigade and Pukovnik Stjepan Zuech’s 9^th Garrison Brigade; and as the Bulgarian component the 61^st Regiment of General Major Simeon Simov’s 24^th Division.

Under the tactical command of SS-Obergruppenführer und General der Waffen-SS Artur Phleps’s V SS Gebirgskorps, the Axis forces first attacked the 36^th and 38^th Divisions in eastern Bosnia beginning 3 August as part of the effort to compress them and then cut them off around Foča on the Drina river. The Axis forces then surrounded and closed on the partisan main body in the Durmitor mountain area in Montenegro beginning on 12 August, and this was the main phase of the operation. In the intense fighting which followed, the partisans suffered heavy losses, even though they had been ordered by Tito to avoid contact with the Axis forces and move to the south-east. By the last week of August the weakened, but still intact, divisions of the I Proletarian Corps had broken out of Montenegro and pushed into south-western Serbia, and so opened the way for the eventual establishment of a major junction with the Soviet forces advancing into Romania.

On 22 August a force of 36 Allied transport aircraft, escorted by 50 fighters, undertook the evacuation to Bari in southern Italy of some 1,059 partisans (most of them wounded), 16 Allied air crew who had been shot down earlier over Yugoslavia, and three members of the Allied military missions. The transports flew in to a small strip near the village of Gronja Brezna about 12.5 miles (20 km) to the south of the Durmitor mountain region in full view of nearby German patrols.

There are no estimates for the Axis losses, but the Germans claimed 2,052 partisans counted dead, plus a further 3,370 estimated dead and wounded, and 242 taken prisoner. The operation’s result, while satisfactory inasmuch as the partisans had been stopped from moving into Macedonia in force, was limited by the lack of a German airborne battalion, hampered by a shortage of fuel, to drop onto, take and hold the chokepoints through which the partisans had to move. Moreover, the worsening situation in Bulgaria and Romania led the Germans to withdraw the 1^st Gebirgsdivision as soon as the main phase of the operation had been completed, so preventing any chance of exploitation. Moreover, the partisan units which had managed to move to the east were now in position to threaten all road and rail transportation to the north and also to Belgrade.