Battle of Issus
As with most ancient battles, significant carnage occurred after the battle as the pursuing Greeks slaughtered their crowded, disorganized foe. Arrian notes Ptolemy mentioning that while pursuing Darius, Alexander and his bodyguards came upon a ravine which they easily crossed on the piled up bodies of dead Persians. It was a decisive victory for Alexander.

Aftermath: The Battle of Issus was a decisive Hellenic victory and it marked the beginning of the end of Persian power. It was the first time the Persian army had been defeated with the King (Darius III at the time) present. After the battle, the Hellenes captured Darius' wife, Stateira I, his daughters, Stateira II and Drypetis, and his mother, Sisygambis, all of whom had accompanied Darius on his campaign. Alexander, who later married Stateira II, treated the captured women with great respect.
Later, the Spartan king Agis III recruited the Greek mercenary survivors of the Battle of Issus who had served in the Persian army, a force of 8,000 veterans, and used them in his fight against the Macedonians. In the summer of 331 BC, Agis defeated Coragus, the Macedonian general in command of the Peloponnese and the garrison of Corinth, but was finally defeated at the Battle of Megalopolis.

Depictions of the battle: German Renaissance painter and printmaker Albrecht Altdorfer (c. 1480–1538): The Battle of Alexander at Issus (1529).
The Battle of Issus (c. 1599–1600) by Jan Brueghel the Elder (1568–1625) hangs in the Louvre.

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References: 

Sources: 

Ancient: Diodorus Siculus (90–30 BC). Bibliotheca Historica. (Reference 6)
Quintus Curtius Rufus (AD 60–70). Historiae Alexandri Magni.
Plutarch (AD 75). The Life of Alexander the Great, Parallel Lives.
Arrian (AD 86–146). Anabasis Alexandri.
Junianus Justinus (3rd century). Historiarum Philippicarum libri XLIV.

Modern: Delbrück, Hans (1920). History of the Art of War. University of Nebraska Press. Reprint edition, 1990. Translated by Walter, J. Renfroe. 4 Volumes.
Engels, Donald W. (1978). Alexander the Great and the Logistics of the Macedonian Army. Berkeley/Los Angeles/London.
Fuller, John F. C. (1960). The Generalship of Alexander the Great. New Jersey: Da Capo Press.
Green, Peter (1974). Alexander of Macedon: A Historical Biography.
Moerbeek, Martijn (1997). The battle of Issus, 333 BC. Universiteit Twente.
Rogers, Guy (2004). Alexander: The Ambiguity of Greatness. New York: Random House.
Warry, J. (1998), Warfare in the Classical World. ISBN 1-84065-004-4.
Welman, Nick. Army. Fontys University.

External links: 
Livius.org: Issus (333 BCE)