It was not till the summer of B.C.331, twenty months later that the Macedonian forces were set in motion towards the interior of the Empire.

More than a year and a half was consumed in the reduction of Phoenicia, the siege of Gaza, and the occupation of Egypt.

Alexander, apparently, was confident of defeating Darius in a pitched battle, whenever and under whatever circumstances they should again meet; and regarded as the only serious dangers which threatened him, a possible interruption of his communications with Greece, and the employment of Persian gold and Persian naval force in the raising of troubles on the European side of the Egean.

He was therefore determined, before he plunged into the depth of the Asiatic continent, to isolate Persia from Greece, to destroy her naval power, and to cripple her pecuniary resources.

The event showed that his decision was a wise one.