On June 9, 1915, the capture of Gradisca completed the Italian control of the lower Isonzo, and Cadorna prepared for a general attack on all the strongholds guarding Trieste. Of these the most important were the Carso tableland on the south, Gorizia barring the river-valley of the Vipacco between the Carso and the foothills of the Julian Alps, the fortified system of heights north of Gorizia surrounding the town of Tolmino, and the great intrenched camp of Tarvis above Tolmino extending to Malborghetto and the other Alps of Carinthia. These fortified points had to be attacked generally or not at all. Any attempt to mass an army against any one of them would have spelled disaster, for the Italians would have been flanked by Austrian forces from the north or south. A properly defined advance against Trieste called for a simultaneous thrust at Tolmino and the Tarvis fortress commanding the road to Vienna. The Austrians had been strengthening Tarvis ever since 1859, after Napoleon III overthrew the Austrians in the battles that freed Lombardy. The Austrian fortresses were again strengthened after the siege of Port Arthur had demonstrated the power of high-explosive shells, and again in 1910 when the Teutonic allies made their great discovery that their new giant howitzers laughed at modern defense works of steel and concrete. In remodeling her Alpine strongholds Austria selected positions on the plateau for systems of earthworks containing mobile siege guns. The key to this immensely strong Austrian line of defense was the railway town of Plava on the eastern bank of the Isonzo under the wooded heights of the Ternovane Forest. Plava was in a salient occupying about the middle of the Austrian line. Here, on the night of June 17, 1915, the Italians began their general offensive by an attack from Mount Korada on the opposite side of the river. Under cover of darkness the Italian sappers built a pontoon bridge, and the Bersaglieri crossed and carried the town and the surrounding heights at the point of the bayonet. The Austrians realized the importance of the position and quickly returned to a violent counterattack. The Italians threw all their available men into the gap, and a great battle raged on the edge of the highlands east of the river. The Austrians had the advantage of position, for their forces could be massed in the woodland out of sight of the Italian aviators. But, on the other hand, the Italian batteries on Mount Korada were able to pour a plunging fire into the lower tableland; and due mainly to the aid of their artillery the Italian troops drove back the enemy and maintained the ground won by the first dash.