By the time he was 29 years old, Alexander the Great was the emperor of Greece, the emperor of Persia by virtue of defeating Xerxes on the plain of Arbella, and the Pharoah of Egypt, and was now knocking on the door of India. His was a warrior’s heart. He, like Napoleon, spent his entire reign waging warfare and purloining territory, and his upwards of 50,000 hoplite and cavalry were with him as career soldiers. They worshipped him and would follow him to gates of Hell because he made them heroes. Historically, the Greek heroes of the past were Agamemnon, Achilles, and Hercules. So conquering was assumed to be the manifestation of heroism. It was a natural next pursuit to follow a living hero into battle. Alexander’s men wielded authority at the business end of swords and spears and arrows. On the plains of Arbella in present day Iraq, the Alexander’s Greek army shut the door on the Persians plans to invest in Anatolia. By the time this battle opened, his empire began in North Africa, engulfed almost the entire Mediterranean all the way East to Afghanistan.
Alexander’s force was around 6,000 hoplites at Hydaspes, armed with spears and swords, small round shields and some body armor and helmets, plus around 5,000 cavalry, and especially a force of mounted archers that Alexander assembled reprising the Parthians he met in Central Asia. Some sources say that Alexander had a far larger force of 20,000 infantry and 9,000 cavalry if you included the mercenaries he hired. To conquer India, Alexander knew he had to move decisively against King Porus, a blooded soldier King who occupied the region of Punjab in modern day Pakistan. He brought his far outnumbered force to the banks of the Indus River and the Hydaspes River. But then, being outnumbered is not being outmatched. At least in Alexander’s battle.
Alexander crossed the first river and had to face King Ambi of the Taxila city-state. Ambi made a deal and decided to pay tribute and allow the Greeks to allow them to pass. It was a wise decision as Alexander’s cruelty to resistant leaders preceded him.
But the Greeks had yet another river to cross and that was the Hydaspes, a seasonal, wide and anfractuous river that would have challenged any army to cross. When Alexander brought his forces upon the western banks of the Jhelurn River and faced the 60,000 Indians, and upwards of 200 war elephants, King Porus stared back across the expanse. The Greeks had really not seen war elephants and simply relied on their common sense about animals when combatting them. That said, the Macedonian infantry formations were correct when guessing that long spears would dissuade some elephants from charging.
Alexander saw that larger force across the Hydaspes and made feints as if he was going to cross. When he did, Porus maneuvered his army to oppose him. When Alexander moved cavalry south, Porus moved cavalry south. This went on for days. Alexander’s fossicking around the banks told the Indian pickets that they were looking for a place to cross. After a while Porus stopped matching the Greeks and realized they had not found a path yet.
Each evening Alexander had his troops build fires and make noise, training the Indian picket guards to accept this din as a common level of noise. All of this was a way to reduce the anxiety in the Indian force. Of course he planned to move his force to a ford wherever he might find it and cross with enemy unawares.
The Greeks even ordered pack trains of grain, which would indicate to Porus that the Greeks were not going to cross until another season when the tide and depths was more favorable.
Porus and his forces put their guard down.
Alexander meanwhile found a crossing of the Hydapses around 17 miles north of Porus’ force, but he kept a few thousand troops across the river from Porus and instructed them to continue to make noise each night and make campsites effulgent with fires and cooking. In the middle of the night, Alexander and his forces secreted themselves into the cold brisk chest deep Hydapses, and as he was crossing, a violent storm broke out, further cloaking his movement from Indian pickets. The Greeks made it to shore, many of them floating on goat hide balloons. Then they discovered that this was just an island in the middle of a river; in fact, it is the first of two. So the Greek Army had to make two crossings. Ten next morning, soaked and cold and exhausted they headed south. Two India pickets saw the Macedonian army coming and raced to warn Porus. Porus sent a small reconnaissance force but the Greek cavalry immediately cut them down.
Porus made the mistake of moving the cavalry from his right flank to is left. Alexander ordered a contingent of his cavalry to run around the Indian forces and hit their cavalry from behind as it settled into position on their left flank. Simultaneously he ordered the pinning force of Greek cavalry he left in front to hit them head on. The Indian cavalry was overwhelmed in this pincer maneuver and fled the battlefield. In the ensuing battle the Greeks managed to use arrows to create confusion in the small Indian cavalry and destroyed them while they were trying to reconstitute as a force behind the Indian infantry. The elephants were in disarray, dissuaded by pikes and spears. As the Indian cavalry rode off, the Greeks surrounded the Indians and in a four or five hour Cannae like encirclement fight, killed upwards of 30,000 and took the rest prisoner. Porus was given his freedom because Alexander so admired him for his courage. In fact, Porus was allowed to keep his kingdom and the remainder of his forces if he accepted the offer to be a satrap in The Alexandrian empire.
Sources: War In The Classical Age, History Channel, BBC Video, Wiki.