The year 1119 marks the definite beginning of the long struggle between the Communes of Pisa and Genoa. The immediate cause of the war seems to have been the privilege of Gelasius II; but, in no circumstances could hostilities have been long averted. The Tyrrhenian Sea was too narrow for more than one mistress; the interests and ambitions of the two cities clashed continually, and, thanks to the prestige which Pisa's earlier triumphs had won her, Genoa was being steadily relegated to a position of commercial inferiority. There was but one alternative: she must either destroy the sea-power of Pisa or be herself destroyed by it. And here her geographical position served her well. The barren mountains which cut her off from territorial extension inland also formed a well-nigh impregnable rampart against attack; strategically she was to all intents and purposes an island, and, once she had established her authority over the two Riviere, the only outlet which was left to her was the sea. Nature had forced upon her an unity of aim which did much to insure success. The case of Pisa was very different. Without a single great natural harbour and with no mountain barrier on the landward side, she was continually tempted to territorial acquisitions and continually distracted by land warfare.