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 was mostly absent: in all, no more than 3,000 Ottoman Christian rebels joined the Serbian army, which was not enough to make a 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 attempts to push the Ottomans back on the eastern border failed with a decisive defeat
at Veliki Izvor (6–11 July). == Ottoman offensive ==