The battle of Hydaspes - Alexander the Great conquest of India

The Battle of the Hydaspes River was fought by Alexander the Great in 326 BC against King Porus of the Paurava kingdom on the banks of the Hydaspes River (Jhelum River) in the Punjab near Bhera. The battle resulted in a complete Macedonian victory and the annexation of the Punjab, which lay beyond the confines of the defeated Persian empire, into the Alexandrian Empire.
Alexander's decision to cross the monsoon-swollen river despite close Indian surveillance, in order to catch Porus' army in the flank, has been referred as one of his "masterpieces". Although victorious, it was also the most costly battle fought by the Macedonians. The resistance put up by King Porus and his men won the respect of Alexander, who asked Porus to become a Macedonian satrap. The battle is historically significant for opening up India to Greek political (Seleucid, Greco-Bactrian, Indo-Greek) and cultural influences (Greco-Buddhist art), which continued to have an impact for many centuries.
The battle took place on the east bank of the Hydaspes River (now called the river Jhelum, a tributary of the river Indus) in what is now present-day Punjab. Alexander later founded the city of Nicaea on the site; this city has yet to be discovered. Any attempt to find the ancient battle site is complicated by considerable changes to the landscape over time. For the moment, the most plausible location is just south of the city of Jhelum, where the ancient main road crossed the river and where a Buddhist source mentions a city that may be Nicaea. The identification of the battle site near modern Jalalpur/Haranpur is certainly erroneous, as the river (in ancient times) meandered far from these cities.  the answer to where the Haydaspes River is located is in the Eastern, at the edge of the Persian war.
After Alexander defeated the last of the Achaemenid Empire's forces under Bessus and Spitamenes in 328 BC, he began a new campaign to further extend his empire towards India in 327 BC. Whilst possessing a much larger army, at the battle, an estimated 40,000 infantry and 5,000 cavalry crossed the river in time to engage the enemy. Depending on the source, Alexander was outnumbered somewhere between 3:1 and 5:1.
The primary Greek column entered via the Khyber Pass, but a smaller force under the personal command of Alexander went through the northern route, taking the fortress of Aornos (modern-day Pir-Sar) along the way—a place of mythological significance to the Greeks as, according to legend, Herakles had failed to occupy it when he campaigned to India.
In early spring of the next year, he combined his forces and allied with Taxiles (also Ambhi), the King of Taxila, against Taxiles' neighbor, the King of Hydaspes.
When Porus reached the point where Alexander's army was arrayed, he deployed his forces and commenced the attack. The Indians were poised with cavalry on both flanks, their center comprising infantry with elephants towering among or before them in equal intervals. The elephants caused much harm to the Macedonian phalanx, but were eventually repulsed by the dense pikes of the phallangitai, wreaking much havoc upon their own lines.
Alexander started the battle by sending horse archers to shower the Indian left cavalry wing. Then, he led the charge against the weakened Indian wing. The rest of the Indian cavalry galloped to their hard pressed kinsmen but at this moment, Coenus's cavalry contingent appeared on the Indian rear. The Indians tried to form a double phalanx, but the necessary complicated maneuvers brought even more confusion into their ranks making it easier for the Macedonian horse to conquer. The remaining Indian cavalry fled among the elephants for protection, but the beasts were already out of control and would soon retreat exhausted from the field, leaving the rest of Porus's army encircled by the Macedonian horse and phalanx. At this time, the phallangitai locked their shields and advanced upon the confused enemy. Porus, after putting up a brave fight, surrendered and the battle was finally over. According to Justin, during the battle, Porus challenged Alexander, who charged him on horseback. Alexander fell off his horse in the ensuing duel, his bodyguards carrying him off and capturing Porus.
According to Arrian, Macedonian losses amounted to 310. However the military historian J.F.C. Fuller sees as "more realistic" the figure given by Diodorus of about 1,000, a large number for a victor, yet not improbable, considering the partial success of the Indian war elephants. Indian losses amounted to 23,000 according to Arrian, 12,000 dead and over 9,000 men captured according to Diodorus. The last two numbers are remarkably close, if it is assumed that Arrian added any prisoners to the total Indian casualties. Around 80 elephants were captured alive. Two sons of Porus were killed during the battle, as well as his relative and ally Spitakes, and most of his chieftains.
The bravery, war skills and princely attitude of Porus greatly impressed Alexander, who allowed him to rule Hydaspes in Alexander's name. Wounded in his shoulder, standing at over 2.1 m (7 feet) tall, he was asked by Alexander how he wished to be treated. "Treat me, O Alexander, like a king" Porus responded. Alexander would indeed treat him like a king, allowing him to retain his kingship. The Macedonian king of most of the known world founded two cities in this region, one at the spot of the battle called Nicaea (Greek for Victory) in commemoration of his success and one on the other side of the Hydaspes called Alexandria Bucephalus, to honor his faithful steed, which died soon after this battle. In 326 BC, the army of Alexander the Great approached the boundaries of the Nanda Empire. His army, exhausted from the continuous campaigning and frightened at the prospect of facing yet another gigantic Indian army, demanded that they should return to the west. This happened at the Hyphasis (modern Beas), the exact spot being believed to be at 'Kathgarh' in Indora tehsil of Himachal Pradesh with nearest rail head at Pathankot, Punjab. Alexander finally gave in and turned south, along the Indus, securing the banks of the river as the borders of his empire.
Πηγη: http://en.m.wikipedia.org/wiki/Battle_of_the_Hydaspes