On the other hand the presumption that the Isonzo sector had a degree of vulnerability was found correct, and along the Isonzo line the real Italian offensive from the beginning continued to be directed. The Isonzo is roughly about three miles into Austria, beyond the political boundary. But it is the true military boundary between Italy and Austria, and it was always regarded by the Austrians as their first line of defense. For almost its entire length, as far south as Salcaro, about four miles north of Gorizia, the Isonzo River runs through a deep gorge and is easily defended. From Salcaro to the sea it issues from the gorge into a more level country--the plateaus of Gorizia and of Carso--although even the southern part of the line is dominated by a series of elevations in supporting distance of each other. Until the line of the Isonzo was forced, Trieste and the entire Istrian Peninsula might be regarded as safe. Although the line of the Isonzo was, as has been shown, the only feasible line on which Italy could advance, no serious offensive could be attempted until the outlets from the Trentino were thoroughly and effectively stopped up. For Italy to have advanced in the Carso, with her rear open to attack by the Austrians coming through the Tyrolean passes, would have been foolhardy. Italy's first step, therefore, was to start a simultaneous forward movement through every pass from Stelvio on the west to the pass near Pontebba on the north. These movements naturally were of an offensive nature, although they were really for a defensive purpose. No attempt was made to advance any distance through the western passes. The Italians were content to take the fortifications guarding the entrance and to seize heights commanding the approaches. On the south and east of the Trentino, however, the operations took on a more extended and, for the Austrians, a more serious aspect. On the south the principal efforts were directed against Riva and Rovereto. The operations against Riva, which is situated at the head of Lake Garda, were directed along the valley of the Ledro and thence along the Tonale River, a small stream connecting Lake Ledro and Lake Garda. At the same time the Italians pushed with energy down the Val Sugana, which leads directly to Trent. The advance was pushed to a point where there was no possibility of the Austrians coming through, and there the Italian forces rested.