That the Bulgarians and Germans were not expecting an assault on the Dobropolje is hardly surprising, given the nature of the terrain. Alan Palmer describes it thus, in ‘Defeat of Bulgaria – The Central Powers Begin to Crack’ (published in History of the First World War No. 107 by Purnell for BPC Publishing Ltd, London, in cooperation with the Imperial War Museum):
Ninety miles north-west of Salonika the Moglenitsa Mountains form a line of bare summits, some 7000 feet high, which marked the frontier between Serbia and Greece. One of the peaks, the Kajmakčalan, had been successfully stormed by the Serbs in September 1916; but the Bulgars had continued to base their defences upon a line of mountains further to the north-east. Their key position was a peak known as the Sokol (Falcon) which was linked to a twin peak, the Ventrenik (Wind Swept One) by a formless and broken ridge six miles in extent, the Dobropolje. Three miles farther northwards the Bulgarian second line of defence ran through a slightly higher mountain, the Kozyak.
When General Franchet d’Espèrey in arrived in Salonika he studied the plans of his predecessors, Sarrail and Guillamaut, but as they assumed that a frontal assault up the Vardar valley was the only way of penetrating the Bulgarian lines – which was likely to be expected and heavily defended – he decided to look elsewhere. Serbian and British commanders were confident that the war-weary Bulgarians would crack in the face of a sustained attack so, without waiting for direct instructions from Paris, Franchet d’Espèrey set out on a tour of the French and Serbian positions in the mountains and found a place where the Serbs were confident that a surprise attack would break the Bulgarian line: Dobropolje.
Within three weeks of his arrival at Salonika, General Franchet d’Espèrey had his basic plan for the offensive; persuading governments and, especially overcoming the objections of “westerners’ would take longer and so would the preparations. Fortunately Franchet d’Espèrey set these in train without waiting for authorisation and hundreds of labourers were soon at work opening up routes from supply bases around Salonika to the Moglenitsa Mountains. By the end of August French sappers and pioneers, with tackle and tractors, had overcome one of the main obstacles, by getting twelve heavy guns in position to dominate the Dobropolje positions at over 7000 feet! Two more batteries were setup on similar peaks, almost as high, to command the second line of defence around the Kozyak. Of course all these preparations had to be undertaken with care, cunning and camouflage nets to avoid alerting the Bulgarians and Germans to the focus of the coming assault.