Serbian offensive
On July 2, 1876, all four major Serbian armies crossed the border into the Ottoman territory, hoping to inspire massive popular uprisings of the local Christian population. However, local support for the Serbian liberation from Ottoman rule was mostly absent: in all, no more than 3,000 Christian rebels joined the Serbian army, which was not enough to make the difference.
On the western border, a Serbian army of about 20,000 men crossed the Drina river and invaded Bosnia, but was unable to take Bijeljina and retreated back to Serbia after several days.
Further south, another Serbian army of about 11,500 men marched on Sjenica and Novi Pazar, but was defeated in the first Battle of Kalipolje and forced to retreat to Javor mountain, where it fought several defensive battles till the end of the war.
On the eastern border, another Serbian army of about 25,000 troops invaded Ottoman Bulgaria and advanced on Kula, but was quickly outmaneuvered by the Ottoman army from Vidin with 10,000 troops, who then went on the offensive into Serbian territory and took Veliki Izvor near Zaječar on July 3. The Serbian Eastern Army was immediately forced to go on defensive, and the Serbian High Command sent orders to the Serbian Army of the South, made up of 68,000 men to reinforce it, aborting the main Serbian offensive on the southern border, aimed on Niš, after only a few days. However, Serbian attempt to push the Ottomans back on the eastern border was heavily defeated in the battle of Veliki Izvor (July 6-11th).[2][7][4][8]
Ottoman offensive
After the Ottoman victory in the battle of Veliki Izvor, the Serbian eastern and southern armies were pushed back (July 15-August 6), and the Ottoman forces took Zaječar and Knjaževac. However, the Ottoman eastern army was unable to push further into Serbia proper on the eastern front, due to the very rough and impassable mountain terrain of the eastern Serbia. In fact, although the Ottomans have successfully occupied the Timok Valley the only route from there into the rest of Serbia was through but two mountain passes (Čestobrodica and Bovan Gorge near Sokobanja), which were already fortified and heavily defended by the Serbian eastern army. So the main Ottoman army (some 60,000 strong) had to attack from the south, from its base at Niš through the Morava Valley.[2][4][7][8]
However, the Serbian southern border and the Morava Valley were defended by heavily fortified towns of Aleksinac and Deligrad, which was also the base of the main Serbian army, some 68,000 strong, with more than 100 pieces of artillery. So the first Ottoman attack on Aleksinac (on August 23) was soundly beaten back in the Battle of Šumatovac. Realising that the main Serbian fortifications in Aleksinac (on the eastern bank of the Morava) were too strong to breach, the Ottoman command sent the bulk of its forces on the western bank of the Morava, where Serbian fortifications were few and far between. So, on the western bank of the Morava, Ottoman forces successfully beat the Serbian army back in the battles of Adrovac (September 1), Krevet (October 1) and Veliki Šiljegovac (October 19). After the fall of Đunis on October 29, the last Serbian fortification on the western bank of the Morava, the Serbian southern front was effectively breached.
