These were the two great contending forces that were set in motion by the departure of Baron Giesl, the Austro-Hungarian Minister, from Belgrade, on July 25, 1914. On the same day the Prince Regent Alexander signed a decree ordering the general mobilization of the Serbian army. Three days later, on July 28, 1914, Austria declared war. By that time Serbia was in the midst of her mobilization. That the Austrians, who had the advantage of having taken the initiative, and who had presumably chosen their own time for the opening of hostilities, did not immediately take full advantage of their favorable situation has caused much surprise among impartial military critics. On the same day that they declared war they had the opportunity to hurl their troops across the Danube and take Belgrade with practically no opposition. Apparently they were not ready; from that moment the difficulties that would have attended such a movement increased hourly. A force of 20,000 men was raised almost immediately for the defense of Belgrade. To meet this opposition the Austrians had, on the evening of the day war was declared, July 18, 1914, only one division concentrated between Semlin and Pancsova, opposite Belgrade--a force that was hardly sufficient to take the Serbian capital. Two days later an army corps would have been needed for the enterprise, for by this time the Serbian army had begun concentrating considerable numbers within striking distance of the capital. Thus the first opportunity was lost by the tardiness of the Austrians to act. It is presumed that the reader has already studied the description of this theatre of the war presented elsewhere in this work. Aside from that, the movements that follow should only be traced with the aid of a map. Written words are inadequate to give a concrete picture of the field of operations. The Austrian General Staff realized the difficulties of crossing the Danube. Its general plan, probably prepared long before, contemplated a main attack that should begin from another quarter. The Austro-Serbian frontier, almost 340 miles in extent, is formed on the north by the Save as well as by the Danube, and on the east and southeast by the Drina River. These two smaller streams abound in convenient fords, especially in summer. To many of these points on the northeastern frontier Austria had already constructed strategic railways. Moreover, the Austrian territory throughout this section is so mountainous and well timbered that large forces of troops could be well screened from observation, whereas the country opposite Belgrade is flat and bare.