The offensive began on 17 August 1916 with the Bulgarians taking Florina (Lerin), Banitsa and Kastoria. The advance, however, soon ran into difficulty and slowed down considerably due to the increased Serbian resistance. The fighting was especially heavy on the bare rocky slopes of the Chegan Mountain and the Nidže. The Serbians were constantly reinforced with new artillery and fresh troops thanks to railway that reached the battlefield while the Bulgarians soon began depleting their ammunition stocks. This and the slow advance forced the Bulgarian high command to call off all attacks on 27 August and order the forces to dig in on the occupied positions between Lake Ostrovo, Lake Petrovsko and along the ridges of the Nidže. For the next several days the Bulgarian positions were subjected to heavy artillery fire and few Serbian attacks that were repulsed. The so-called Chegan Offensive, also known as the Lerin Offensive, had failed. It failed to influence Romania, which entered the war on the side of the Allies but also failed to achieve its final military objective to take the Chegan village and the pass north of Lake Ostrovo.