The Battle of Issus, was a battle between the invading Macedonian troops, led by Alexander the Great against an Persian army lead by Darius III during Alexander's Persian Campaign. It occurred in southern Anatolia, in November 333 BC and was a devastating victory Alexander's. Darius took personal command of his army, after the Macedonians soundly defeated the Persian satraps of Asia Minor (led by the Greek mercenary, Memnon of Rhodes) at the Battle of the Granicus. He gathered reinforcements and led his men in a surprise march behind the Macedonian advance to cut their line of supply. This forced Alexander to countermarch, setting the stage for the battle near the mouth of the Pinarus River and the town of Issus.

Darius forces are approximated between 25,000 and 100,000 whereas Alexander commanded 40850.

During the battle, Alexander's Companion cavalry led a direct assault against Darius who fled from the battlefield. Alexander then saw his left flank and center in trouble, let Darius flee, and crashed into the rear of the Greek mercenaries. The Greek mercenaries broke up. The Persians saw that their Great King had gone and that the battle was being lost, and they abandoned their positions and fled in full rout. The Macedonian cavalry pursued the fleeing Persians for as long as there was light. As with most ancient battles, significant carnage occurred after the battle as pursuing Macedonians slaughtered their crowded, disorganized foe. Arrian notes Ptolemy I mentions that, while pursuing Darius, Alexander and his bodyguards came upon a gap which they effortlessly crossed on the bodies of dead Persians. It was a decisive victory for Alexander.

the Greeks lost 7,000 men that day whereas the Persians approximately 20,000.