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Why did the Achaemenid Empire collapse after Alexander the Great invaded?

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Relief of three Achaemenid warriors, known as the Ten Thousand Immortals, depicted in profile on glazed bricks with patterned robes and bows, from the Palace of Artaxerxes at Susa.
Achaemenid Empire Frieze of Archers from the palace of Darius I at Susa, c. 510 bce; some believe the warriors are among the Ten Thousand Immortals.

The collapse of the Achaemenid Empire following the invasion by Alexander the Great was the culmination of long-term internal instability and military failure.

By the time Alexander crossed the Hellespont in 334 BCE, the empire had been weakened by decades of palace intrigue, frequent assassinations, and persistent rebellions, such as those in Egypt and among the satraps. The reign of Darius III was marked by a lack of preparation to resist the Macedonian advance.123

Alexander’s success was driven by his highly disciplined, balanced military force, which included the elite Companion cavalry and a phalanx of heavy infantry. After decisive defeats at the Battle of Issus and the Battle of Gaugamela, Darius III fled his own forces. His eventual assassination by the satrap Bessus in 330 BCE signaled the final dissolution of the empire founded by Cyrus the Great.41