'Trumpf' was an Axis operation against the newly formed 21st, 22nd, 24th and 25th 'Serb' Divisions, totalling some 8,000 men, of Marshal Josip Broz Tito’s partisan forces in Axis-occupied southern Serbia (more specifically the area of Kopaonik and Jastrepca and in the Toplica and Jablanica valleys) with the objective of neutralising their threat to the Axis forces in central and northern Serbia, and of preventing them from joining forces with the Yugoslav Operative Group of Divisions trying to break into Serbia (about 10/19 July 1944).
'Trumpf' was joint operation by substantial German forces (5th SS Polizeiregiment, elements of the 1st Freiwilligen Polizeiregiment and 5th Freiwilligen Polizeiregiment, elements of the 15th Panzerregiment, and elements of the Russisches Schutzkorps) backed by Bulgarian units (parts of General-major Anton Baltakov’s 22nd Division, General-major Hristo Kozarev’s 27th Division and Polkovnik Ivan Popov’s 29th Division), Serb collaborationist units (elements of the Srpski Dobrovoljacki Korpus and Srpska Drzavna Straza), the Rasina-Kopaonik Group of the Assault Corps commanded by Dragutin Keserović, the 2nd Kosovo Corps and the elite Četnik 4th Group of the Assault Corps commanded by Radoslav Račić.
This last group had arrived from western Serbia. The Četniks constituted some 10,000 men out of the Axis force of 80,000 men co-ordinated by Germans, who also supplied the Četniks with munitions and some weapons.
'Trumpf' was in fact the first part of a three-phase offensive against the partisans known to the latter as the 'Topličko-jablanička operacija'. In this first phase, the German leadership decided to attack from the valley of the Toplice river, the Kopaonik mountain area and the valley of the western Morava river, thereby paralysing the capability of the 21st, 22nd, 24th and 25th Divisions, as well as other independent partisan units and forces, to move and to operate, in the process driving the partisan formation and units back into the mountain areas between Toplice and Jablanica. The main problem for the attackers was the position of 16th 'Serb' Brigade of the 25th Division and 'Jastrebac' Partisan Detachment , which held positions on the Jastrebac mountain and in the rear of the Axis forces' main semi-circular base line. Thus the Axis operation started on July 10 with a concentric attack on Jastrebac mountain.
The partisan forces suffered no major losses during the night of 10/11 July, and successfully penetrated the line of the Bulgarian 27th Division near Toplice and linked merged with other units of the 25th Division. During this same night, General Koča Popović, the former commander of the I Proletarian Corps, arrived by air on an improvised airstrip on Radan mountain flown, and on 12 July Popović assumed command of the General Staff of Partisan Serbia.
With a view to disrupting the Axis plan and scattering the Axis concentration of force, the General Staff of Partisan Serbia on 13 July instructed 21st Division to cross the Kursumlijske Banje to Lepenc and Brus, and then attack the Axis forces to the north of Toplice. Here the Yugoslav division broke a Četnik brigade and parts of other units, and by 16 July had seized several important heights in the rear of the Axis forces, in the process compelling the 4th Group of the Assault Corps and 2nd Kosovo Corps to make difficult efforts to secure their lines as their former lines between Tulare to Kursumlija were taken over by the Bulgarian 27th Division.
During 18 and 19 July the 21st Division has been pushed out of the Kursumlija area by the combined Axis forces, which thus secured the positions they needed for their planned decisive attack in 'Hallali'.