It was necessary that Alexander and his army eliminate all Persian naval threats in the Aegean and Levant before continuing inland on their campaign. If the Persian leaders realised that Greece was only defended by 13,000 men, there would be a large risk of invasion. Tyre, on the Levantine coast, was expertly defended, well garrisoned and almost impregnable. Additionally, it was a crucial trading port of the Mediterranean and controlled access to India, Carthage, Rome and other great nations across the sea. Alexander, of course, did not want to lead his army into this military headache, but the capture of Tyre was paramount and necessary to the continuation of the invasion.