Germany, after the various Austrian defeats in Serbia, determined to take the command of a new punitive expedition herself, and in view of the co-operation of Bulgaria she had concentrated a powerful Austro-German army, amply supplied with artillery, including guns of the heaviest calibre, in South Hungary under the command of the German Field-Marshal von Mackensen. The invasion of Serbia was carried out by the Austrians and Germans from the north and also from the west (from Bosnia), and by the Bulgarian Army from the east and south-east. The Serbians fought heroically, opposing a desperate resistance on three fronts, and at one moment it seemed as if they might miraculously succeed; perhaps indeed they might have saved themselves, or at least avoided the extreme disaster, if they had only followed the advice of the Allies. But although it soon became known that a new and more formidable attempt was about to be made by the enemy to crush Serbia definitely, the Serbs refused to create a modern defensive system of trenches and wire entanglements, which in a mountainous territory such as that of Serbia would at least have held up the invaders for a considerable time. To the suggestions made by the Allies that these methods be adopted, the Serbs replied with typical Balkan vaingloriousness: “Wire entanglements and trenches are all very well for the Germans and Austrians, for the French, Italians, British or Russians, but we have no use for them; we fight in the open and drive out the enemy.” Their victories over the Austrians had made them lose their heads and forget that these victories were not due solely to their own courage but also, to a considerable extent, to the serious strategical and tactical errors of the Austrian commanders, from General Potiorek downwards, errors which were not repeated by Marshal von Mackensen. The new invasion carried out by the formidable Austro-German Army to which we have referred, and there came also the stab in the back on the part of the Bulgarians.