In the spring of 1916 as the Allies had finished fortifying the area around Salonica and it became obvious that no Bulgarian and German offensive was going to take place, the French and British forces moved up to the Greek border where they established contact with the Bulgarian forces. In the beginning of April the positions of the Eleventh Army were subjected to heavy artillery shelling and field marshal Mackensen requested the 9th "Pleven" infantry division to be given to that army so that it can consolidate its position. General Zhekov agreed and ordered the division to leave the First Army, join the Eleventh Army and concentrate behind the German 4th Corps. By 1 June the Entente forces were additionally strengthened by the arrival of some 115,488 Serbian soldiers, which were organized in three corps-sized formations called armies. The Allies now had over 313,000 troops in the theater which increased the Bulgarian concerns. By July Romania was preparing to enter the war on the side of the Entente which placed Bulgaria in a difficult position facing a possible war on two fronts and in the beginning of August the French and British launched an offensive against the Bulgarian positions at Lake Dojran which convinced the Bulgarian high command that the Allies were preparing a general offensive. To counter these negative developments the Bulgarians planned an offensive of their own with both their flanks aiming to shorten the front line and influence Romania in its decision to enter the war. The advance on the right flank was to be undertaken by the Bulgarian First Army which had the following order of battle in July 1916:  The Germans finally agreed that an offensive was needed and on 12 August general Boyadzhiev received his orders from the headquarters of Army Group "Mackensen". The right wing of the army, consisting of the reinforced 8th infantry division (four and half infantry brigades) was to advance and take Florina while parts of the 3rd division towards the Chegan mountain range and the village of the same name. General Boyadzhiev agreed to attack but had concerns regarding the final results of the offensive because his army was scattered on a 140 km front and lacked enough mountain and heavy artillery. Facing the Bulgarians were the six infantry and one cavalry divisions of the three Serbian armies. 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.