So far (B.C. 333), excepting this, Alexander had taken no step which left any danger in his rear. He had confided every city and country he had traversed to the hands of friends. His advance was our first instance on a grand scale of methodical war,--the origin of strategy. He continued in this course, not proposing to risk himself in the heart of Persia until he had reduced to control the entire coast-line of the Eastern Mediterranean as a base. This task led him through Syria, Phœnicia,--where the siege of Tyre, one of the few greatest sieges of antiquity, delayed him seven months,--and Palestine to Egypt. Every part of this enormous stretch of coast was subjugated. Having in possession, practically, all the seaports of the then civilized world, and having neutralized the Persian fleet by victories on land and at sea, Alexander returned to Syria, marched inland and crossed the Euphrates and Tigris, thus projecting his line of advance from the centre of his base. At Arbela he defeated the Persian army in toto, though they were twenty to his one. Babylon, Susa, Persepolis, and Pasargadæ opened their gates to the conquerer. But Darius escaped. It was now the spring of 330 B.C. Only four years had elapsed and Alexander had overturned the Persian Empire; and though he left home with a debt of eight hundred talents, he had won a treasure estimated at from one hundred and fifty millions of dollars up. [Illustration: CONQUESTS OF ALEXANDER B.C. 334–323] Alexander now followed Darius through Media and the Caspian Gates to Parthia, subduing the several territories he traversed. He found Darius murdered by the satraps who attended him. This was no common disappointment to Alexander, for the possession of the person of the Great King would have not only rendered his further conquests more easy, but would have ministered enormously to his natural and fast-growing vanity. He pursued the murderers of Darius, but as he could not safely leave enemies in his rear, he was compelled to pause and reduce Aria, Drangiana, and Arachosia. Then he made his way over the Caucasus into Bactria and Sogdiana, the only feat which equals Hannibal’s passage of the Alps. His eastern limit was the river Jaxartes, the crossing of which was made under cover of his artillery,--its first use for such a purpose. The details of all these movements are so wonderful and show such extraordinary courage, enterprise, and intelligence, such exceptional power over men, so true a conception of the difficulties to be encountered, such correct judgment as to the best means of overcoming them, that if the test should be the accomplishment of the all but impossible, Alexander would easily stand at the head of all men who have ever lived. He now formed the project of conquering India, and, returning over the Caucasus, marched to the Indus and crossed it. Other four years had been consumed since he left Persepolis. It was May, B.C. 326. [Illustration: BATTLE OF THE HYDASPES B.C. 326]