 In the beginning due to the harsh weather, strong resistance of the defenders and the tough terrain the Bulgarian advance was slow but because defenders were greatly outnumbered, there was a Bulgarian breakthrough near Pirot in 10 days and the Serbs retreated to the Timok.The battle continued for 27 days and the Bulgarians penetrated up to 90 km deep into the Serbia's territory.The Serbs lost 6,000 men; 60 guns and a large amount of military equipment.