Austria was in no happier case. Her invasion of Serbia which had opened the flood-gates of war had been almost submerged in the torrent, and the punitive expedition she had planned had brought punishment mainly upon herself, and that not merely at the hands of Serbia's powerful patron, but at those of the little people who were to be chastised. The early fighting was of a desultory character, and Austria's two first-line corps having been withdrawn to meet the Russians, the Serbs and Montenegrins made a combined effort on 12 August to invade Bosnia and capture Serajevo. No great progress was made, and on the 16th the Austrians retaliated with the capture of Shabatz in the north-west corner of Serbia. But next day the Serbs routed a large Austrian force in the neighbourhood, and the Crown Prince Alexander followed up this victory by another on the 18th against the Austrians on the Jadar, who were seeking, in co-operation with those at Shabatz, to cut the Serbs off from their base. The result was that by the 24th the Austrians were practically cleared out of the country, and Vienna announced that the punitive expedition, which had cost 40,000 casualties and fifty guns, had accomplished its object. A second attempt to achieve it was, however, provoked by the invasion of Bosnia with which the Serbs had supplemented their victory, and by their capture of Semlin in order to stop the Austrian bombardment of Belgrade. On 8 September the Austrians launched an attack across the Drina which forms the boundary between Serbia and Bosnia, and the battle raged till the 17th. Again the Serbs were victorious, though they made no impression on Serajevo and the Austrians retained a foothold on the eastern bank of the Drina; and for six weeks Serbia was left in comparative peace. But at the end of October the entrance of Turkey into the war and the relief afforded to Austria's troops farther north by the increasing activity of Germany in Poland and Galicia encouraged another effort; and under General Potiorek the Austrians began a more ambitious campaign. The Serbian frontier constituted a sharp salient which was indefensible against a superior force, and the Austrians exploited this advantage by extending their front of attack from Semendria on the Danube right round to Ushitza beyond the Drina. Their object was to envelop the Serbs and seize, firstly, Valievo, their advanced base, secondly, Kraguievatz the Serbian arsenal, and, finally, Nish, to which the Serbian Court and Government had withdrawn.