As for the Serbians, their attempts to break through the Austrian positions were fatally hampered by a shortage of ammunition. At one point they did, in fact, succeed in breaking through and then suddenly the ammunition supply came to an end and the Serbians had to retire again, leaving the Austrians to return to the trenches from which they had just been ejected. Up in the northwest the Austrians also held a narrow strip of Serbian territory, along the Drina from Kuriachista up, but with this small exception they were confined to their side of the river until the triangular tract in the northeast of the Matchva Plain was reached, previously mentioned. Along the Save from Parashnitza to Shabatz they had also attempted a southward movement, where they were supported by five river monitors. During the period of comparatively little activity which now followed the Serbians were much worried by these monitors, which patrolled up and down the river at night, throwing their searchlights on and exposing the Serbian trenches. Then, too, they could hurl bombs into the Serbian positions with almost absolute impunity, for whenever the Serbian shells struck the heavy armor of these river fortresses they rolled off harmlessly. On the night of October 22, 1914, the Serbians sent some mines floating down the river, one of which struck a monitor and sank it in deep water. For nearly six weeks through November, 1914, this deadlock continued. But during all this time, the Austrian General Staff was quietly preparing for another grand drive through Serbia. It was then that the 150,000 reserve, previously mentioned, was assigned to General Potiorek's disposal, while his first line was also materially strengthened. Nor did the third invasion begin with any dramatic effort. The pressure was applied gradually, little by little, until the Serbs were finally face to face with the necessity of shortening their lines, if they were not to be broken through. Other causes besides the increasing pressure from the Austrians contributed to the general causes.