In the spring of 331 B. C. Alexander set out again for Persia, where
Darius had been gathering an immense force with which to make a last
struggle for the empire of the world. After traversing Phœnicia and
Northern Syria, Alexander crossed the Euphrates and Tigris, and came out
on the plain near the little village of Gaugamela, to the southeast of
the ruins of Nineveh. Then took place the great and decisive battle of
Arbela, with the Persians, October, 331 B. C.
After receiving the surrender of the other two capitals, Susa and
Persepolis, Alexander spent the year 330 B. C. in conquering the
northern provinces of the Persian Empire, between the Caspian Sea and
the Indus. In 329 B. C. he marched into Bactria, over the mountains now
called the Hindu Kush, caught and slew the traitor Bessus, who murdered
Darius, and advanced even beyond the river Jaxartes. In 328 he was
engaged in the conquest of Sogdiana, between the Oxus and Jaxartes, the
country of which the capital was Maracanda, the modern Samarcand.