Thick
lines show the front on 12, October 1915 and thin ones show it
on 5, October 1915. This map was drawn by Japanese(Imperial)
general staff.
Serbian army held 16 divisions half of which had successfully defeated Australian three times in 1914 under the command of Putnik, chief of general staff.
However, Germans jointed by Austrians and Bulgarians outnumbered Serbs one by two. Putnik deployed 11 in the north but only 5 in the south.
Austrians also feinted on Drina during the summer, which made Serbian difficult to concentrate in the centre. Also in the south Albanian bandits were active instigated by Austrians so that Putnik could not help but organise 25 battalions to silence them.
Putnik's strategy was to knock out Mackenzen coming from north first and turn all armies to south despite the fact that Serbian were inferior in number.
Macedonia is mountainous so that Bulgarians' march was estimated to be slow.
Germans strategic objective was to make a corridor to Constantinople. In this regard it is easy to speculate Germans' real target. Not River Drina but the Danube in the centre. Even if Putnik grabed this idea he seemed to have nothing to counter it.
In the south French Orient Army reinforced by British finished to concentrate 3 divisions in Salonica
According to the observation by Japanese(Imperial) general staff Serbian should concentrate all force in the south because it was easy to defend geographically and meteorogically. Moreover, they expect French to come to succour.
This observation is doubtful because Serbian should be loyal to the Allied cause. They ignited all flames in Europe. If Serbian abandoned the north Germans could acquire the logistics critical for Turkish war efforts without fighting.
Putnik's strategy failed because Bulgarians came so fast. Mackenzen's march was rather slow confirming supply lines and scouting against flank assaults. Bulgarians ignored the manifested declaration of war and dashed with water obstacles betrayed beforehand.
Serbian gave up to resist in the north and retreated to Albania.
However, Putnik was an ingenious general enough to give Germans a severe blow by triple mine line on the Danube and suicide attack boats with one man and explosives. Mackenzen lost 60 thousand out of 15 divisions when crossing the Danube.
It took the whole six days to cross the Danube and when Kaiser visited the crossing point Mackenzen apologised for the loss of soldiers.
Serbian withdrawal to the west was also brilliant to set up suicide attacks by rear guard, which slowed German ,Austrian and Bulgarian pursuit. Bulgarians also suffered 100 thousands loss. On the other way round Serbian lost 264 thousands, two third of total army and all territories.