'Treubruch' was a German blocking operation at Skopje in the southern part of German-occupied Yugoslavia (September 1944).
On 8 September Bulgaria switched sides and declared war on Germany, and immediately dispatched significant forces from Sofia in the direction of the Yugoslav frontier. Lacking the strength to put their ambitious 'Hundessohn' and 'Judas' contingency plans into effect, the Germans hurried Generalleutnant Walter Stettner Ritter von Grabenhofen’s 1st Gebirgsdivision to Skopje, where it operated temporarily under command of General Maximilian de Angelis’s 2nd Panzerarmee to implement 'Treubruch'.
While 5,000 Bulgarians of General-major Asen D. Nikolov’s I Occupation Corps at Bitolj laid down their arms after a brief show of resistance, the Bulgarian garrison at Prilep and Bulgarian units in the Skopje area, which were elements of General-major Konstantin B. Stoyanov’s 5th Army, fought stubbornly. However, the 1st Gebirgsdivision eventually managed to seize and hold the city of Skopje and the vital railway along the Vardar river. Bulgarian units bypassed in the fast German move to the north succeeded in breaking the line at a number of points and had to be driven off by Kampfgruppen formed from troops moving to the north from Greece to join the 1st Gebirgsdivision.