Why was Alexander the Great able to conquer the Persian state?

The reason for the series of victories over the Persians is the excellent tactics and strategy employed by the Macedonian. In the center of the troops were the phalanxes. Light and heavy cavalry attacked from the flanks. The distribution of forces on the flanks was different. Particular hope was pinned on the right flank. Such actions were a kind of gamble. The Persians could rebuild their order and destroy this flank. In addition, the cavalry of the right flank had to make a fairly long maneuver at a considerable distance from the infantry. Macedonian constantly used flanking maneuvers.

Another reason was the transfer of some of the satraps to the side of Alexander. They stopped trusting their ruler and quite openly betrayed him.

The Egyptians, being conquered by the Persian troops, met Alexander as their liberator.