Indian campaign of Alexander the Great
The Indian campaign of Alexander the Great began in 327BC and lasted until 325BC. After conquering the Achaemenid Persian Empire, the Macedonian army undertook an expedition into the Indian subcontinent. Within two years, Alexander expanded the Macedonian Empire to include present-day Punjab and Sindh in what is modern-day Pakistan, surpassing the earlier frontiers that had been established by the Persian conquest of the Indus Valley. Following Macedon's absorption of Gandhara (a former Persian satrapy), including the city of Taxila, Alexander and his troops advanced into Punjab, where they were confronted by Porus, the regional Indian king. In 326 BC, Alexander defeated Porus and the Pauravas during the Battle of the Hydaspes, but that engagement was possibly their Macedonians' most costly battle. Alexander's continued eastward march was leading his army into a confrontation with the Nanda Empire, based in Magadha. According to Greek sources, the Nanda army was five times the size of the Macedonian army; Alexander's troops—increasingly exhausted, homesick, and anxious by the prospects of having to further face large Indian armies throughout the Indo-Gangetic Plain—mutinied at the Hyphasis River, refusing to advance his push to the east. After a meeting with his army general Coenus, during which he was informed of his soldiers' laments, Alexander relented under the conviction that it was better to return. He subsequently turned southward, advancing through southern Punjab as well as Sindh, where he conquered more tribes along the lower areas of the Indus River, before finally turning westward to reach Macedon. Background

Sources 
There are no ancient sources at all giving an Indian account of the campaign, or even mentioning it at all. There is no mention of Porus in the extremely sparse Indian accounts of the period. There are however Indian literary accounts and Indian folktales, possibly written many years later. Of those who accompanied Alexander to India, Aristobulus, Onesicritus, and Nearchus wrote about the Indian campaign. The only surviving contemporary account of Alexander's Indian campaign is a report of the voyage of the naval commander Nearchus, who was tasked with exploring the coast between the Indus River and the Persian Gulf. This report is preserved in Arrian's Anabasis (). Arrian provides a detailed account of Alexander's campaigns, based on the writings of Alexander's companions and courtiers. Arrian's account is supplemented by the writings of other authors, whose works are also based on the accounts of Alexander's companions: these authors include Diodorus (), Strabo (), and Plutarch (). Socio-political conditions in India 

Alexander's incursion into India was limited to the Indus River basin area, which was divided among several small states. These states appear to have been based on dominance of particular tribes, as the Greek writers mention tribes such as the Malloi as well as kings whose names seem to be tribal designations. The Achaemenid Empire of Persia had held suzerainty over the Indus valley in the previous decades, but there was no trace of Achaemenid rule beyond the Indus river when Alexander's army arrived in the region. Strabo, sourcing his information from the earlier writer Eratosthenes, states that the Achaemenid king controlled the area to the west of the Indus. This area (including the Kapisa-Gandhara region) was probably the territory of the Indians, who according to the Greek accounts, fought alongside their overlord Darius III at the Battle of Gaugamela. Greek writings as well as archaeological excavations indicate the existence of an urban economy dependent on agriculture and trade in the Indus basin. The Greeks mention the existence of cities and fortified towns such as Taxila. Arrian mentions that after defeating Porus, Alexander marched eastwards towards the Chenab River, and captured 37 towns: the smallest of these towns had 5,000 or more inhabitants. In the Swat valley, Alexander is said to have seized 230,000 oxen (possibly Zebu), intending to send them to Macedonia for ploughing land. Aristobulus saw rice being grown in paddy fields, Onesicritus reported the existence of a crop called bosmoran (possibly the pearl millet), and Nearchus wrote of "honey-yielding reeds" (presumably the sugarcane). Nearchus also mentions that Indians wore clothes made of cotton. Rock salt was extracted from the Salt Range, and supplied to other parts of India. Some primitive communities existed in the forest, desert, and coastal regions of the subcontinent. For example, Nearchus mentions that people around the Tomeros river (Hingol) subsisted on fishing, and used stone tools instead of iron ones. The Greek writers mention the priestly class of Brahmanas (as "Brachmanes"), who are described as teachers of Indian philosophy. They do not refer to the existence of any religious temples or idols in India, although such references commonly occur in their descriptions of Alexander's campaigns in Egypt, Mesopotamia and Iran. Greek accounts mention naked ascetics called gymnosophists. A philosopher named Calanus (probably a Greek transcription of the Indian name "Kalyana") accompanied Alexander to Persepolis, where he committed suicide on a public funeral pyre: he was probably a Jain or an Ajivika monk. There is no reference to Buddhism in the Greek accounts. Other than their mention of the Brahmanas, the Greek narratives about Alexander's invasion do not directly mention the caste system. Some Brahmanas acted as advisors to local princes: Alexander had groups of Brahmanas hanged in present-day Sindh for instigating the rulers Musicanus and Sambus to revolt against him. The Greek writings attest the existence of slavery in at least two places: Onesicritus describes slavery in the territory ruled by Musicanus, and Aristobulus mentions poor people selling their daughters publicly in Taxila. Aristobulus also observed Sati, the practice of widows immolating themselves on their husbands' pyre, at Taxila. The practice of exposing dead bodies to vultures, similar to the Magian practice of Tower of Silence, was also prevalent in Taxila. Nearchus mentions that Indians wrote letters on closely woven cloth; it is possible that this is a reference to a precursor of the Kharoshthi script, which may have developed from the Aramaic alphabet during the Achaemenid rule. While describing a tribe on the coast of present-day Balochistan, Nearchus mentions that they were different from Indians in "their language and customs", which implies that he associated a particular language with the Indians. This does not mean that the Indians spoke a single language: the language that Nearchus associated with India might have been a lingua franca used for official and commercial purposes. This lingua franca was most probably the Gandhari Prakrit, as the Greek names (e.g. "Taxila" and "Sandrokottus") for Indian people and places seem to be derived from this language (e.g. "Takhasila" and "Chandagutta") rather than Sanskrit (e.g. "Takshashila" and "Chandragupta"). Nearchus attests the existence of medical science in India: he mentions that when the Greek physicians failed to provide remedies for snake-bites to Alexander, the king gathered Indian healers who were also able to cure other diseases and painful conditions. The Greek accounts do not mention any other sciences of contemporary India. Alexander's preparation 

After the death of Spitamenes and his marriage to Roxana (Raoxshna in Old Iranian) in 326BC to cement his relations with his new Central Asian satrapies, Alexander was finally free to turn his attention to India. For Alexander, the invasion of India was a natural consequence of his subjugation of the Achaemenid Empire, as the areas of the Indus valley had long been under Achaemenid control, since the Achaemenid conquest of the Indus Valley . Alexander was only taking possession of territories which he had obtained from the Achaemenids, and now considered rightfully his own. Alexander invited all the chieftains of the former satrapy of Gandhara, to come to him and submit to his authority. Ambhi (Greek: Omphis), ruler of Taxila, whose kingdom extended from the Indus to the Jhelum (Greek: Hydaspes), complied. At the end of the spring of 327 BC, Alexander started on his Indian expedition leaving Amyntas behind with 3,500 horse and 10,000 foot soldiers to hold the land of the Bactrians. Cophen campaign

Alexander personally took command of the shield-bearing guards, foot-companions, archers, Agrianians, and horse-javelin-men and led them against the clansthe Aspasioi of Kunar valleys, the Guraeans of the Guraeus (Panjkora) valley, and the Assakenoi of the Swat and Buner valleys. Alexander faced resistance from Hastin (or Astes), chief of the Ilastinayana (called the Astakenoi or Astanenoi) tribe, whose capital was Pushkalavati or Peukelaotis. He later defeated Asvayanas and Asvakayanas and captured their 40,000 men and 230,000 oxen. Asvakayanas of Massaga fought him under the command of their queen, Cleophis, with an army of 30,000 cavalry, 38,000 infantry, 30 elephants, and 7,000 mercenaries. Other regions that fought Alexander were Abhisara, Aornos, Bazira, and Ora or Dyrta. A fierce contest ensued with the Aspasioi, in the course of which Alexander himself was wounded in the shoulder by a dart, but eventually the Aspasioi lost the fight; 40,000 of them were enslaved. The Assakenoi faced Alexander with an army of 30,000 cavalry, 38,000 infantry, and 30 elephants. They had fought bravely and offered stubborn resistance to the invader in many of their strongholds such as the cities of Ora, Bazira, and Massaga. The fort of Massaga could only be reduced after several days of bloody fighting in which Alexander himself was wounded seriously in the ankle. When the Chieftain of Massaga fell in the battle, the supreme command of the army went to his old mother, Cleophis, who also stood determined to defend her motherland to the last extremity. The example of Cleophis assuming the supreme command of the military also brought the entire population of women of the locality into the fighting. Alexander was only able to reduce Massaga by resorting to political stratagem and actions of betrayal. According to Curtius: "Not only did Alexander slaughter the entire population of Massaga, but also did he reduce its buildings to rubbles". A similar slaughter then followed at Ora, another stronghold of the Assakenoi. Siege of Aornos

In the aftermath of general slaughter and arson committed by Alexander at Massaga and Ora, numerous Assakenians fled to a high fortress called Aornos (not definitely identified but somewhere between Shangla, in Swat, and the Kohistan region, both in northern Pakistan). Alexander followed close behind their heels and besieged the strategic hill-fort. The Siege of Aornos was Alexander's last siege, "the climax to Alexander's career as the greatest besieger in history", according to Robin Lane Fox. The siege took place in April 326BC. It presented the last threat to Alexander's supply line, which stretched, dangerously vulnerable, over the Hindu Kush back to Balkh, though Arrian credits Alexander's heroic desire to outdo his kinsman Heracles, who allegedly had proved unable to take the place Pir-Sar, which the Greeks called Aornis. The site lies north of Attock in what is now the Punjab, Pakistan, on a strongly reinforced mountain spur above the narrow gorges in a bend of the upper Indus. Neighboring tribesmen who surrendered to Alexander offered to lead him to the best point of access. At the vulnerable north side leading to the fort, Alexander and his catapults were stopped by a deep ravine. To bring the siege engines within reach, an earthwork mound was constructed to bridge the ravine. A low hill connected to the nearest tip of Pir-Sar was soon within reach and taken. Alexander's troops were at first repelled by boulders rolled down from above. Three days of drumbeats marked the defenders' celebration of the initial repulse, followed by a surprise retreat. Hauling himself up the last rockface on a rope, Alexander cleared the summit, slaying some fugitivesinflated by Arrian to a massacreand erected altars to Athena Nike, Athena of Victory, traces of which were identified by Stein. Sisikottos, or Saśigupta, who had helped Alexander in this campaign, was made the governor of Aornos. Punjab
After reducing Aornos, Alexander crossed the Indus to begin campaigning in the Punjab region. Battle of the Hydaspes River

The Battle of the Hydaspes River was fought by Alexander in July 326BC against king Porus (possibly, Paurava) on the Hydaspes River (Jhelum River) in the Punjab, near Bhera. The Hydaspes was the last major battle fought by Alexander. The main train went into what is now modern-day Pakistan through the Khyber Pass, but a smaller force under the personal command of Alexander went via the northern route, resulting in the Siege of Aornos along the way. In early spring of the next year, he combined his forces and allied with Taxiles (also Ambhi), the King of Taxila, against his neighbor, the King of Hydaspes. Porus was a regional King in India. Arrian writes about Porus, in his own words:

Porus drew up on the south bank of the Jhelum River, and was set to repel any crossings. The Jhelum River was deep and fast enough that any opposed crossing would probably doom the entire attacking force. Alexander knew that a direct crossing would fail, so he found a suitable crossing, about  upstream of his camp. The name of the place is "Kadee". Alexander left his general Craterus behind with most of the army while he crossed the river upstream with a strong contingent. Porus sent a small cavalry and chariot force under his son to the crossing. According to sources, Alexander had already encountered Porus's son, so the two men were not strangers. Porus's son killed Alexander's horse with one blow, and Alexander fell to the ground. Also writing about this encounter, Arrian adds, 

The force was easily routed, and according to Arrian, Porus' son was killed. Porus now saw that the crossing force was larger than he had expected, and decided to face it with the bulk of his army. Porus's army were poised with cavalry on both flanks, the war elephants in front, and infantry behind the elephants. These war elephants presented an especially difficult situation for Alexander, as they scared the Macedonian horses. Alexander started the battle by sending horse archers to shower the Porus's left cavalry wing, and then used his cavalry to destroy Porus's cavalry. Meanwhile, the Macedonian phalanxes had crossed the river to engage the charge of the war elephants. The Macedonians eventually surrounded Porus's force. Diodorus wrote about the battle tactics of war elephants:

The fighting style of Porus' soldiers was described in detail by Arrian:

According to Curtius Quintus, Alexander towards the end of the day sent a few ambassadors to Porus:

According to Plutarch this was one of Alexander's hardest battles: Plutarch also wrote that the bitter fighting of the Hydaspes made Alexander's men hesitant to continue on with the conquest of India, considering that they would potentially face far larger armies than those of Porus if they were to cross the Ganges River. Porus was one of many local kings who impressed Alexander. Wounded in his shoulder, standing over  tall, but still on his feet, he was asked by Alexander how he wished to be treated. "Treat me, Alexander, the way a King treats another King", Porus responded. Other historians question the accuracy of this entire event, noting that Porus would never have said those words. Philostratus the Elder in the Life of Apollonius of Tyana writes that in the army of Porus there was an elephant who had fought bravely against Alexander's army and Alexander dedicated it to  Helios (Sun) and named it Ajax, because he thought that a so great animal deserved a great name. The elephant had gold rings around its tusks and an inscription was on them written in Greek: "Alexander the son of Zeus dedicates Ajax to Helios" (ΑΛΕΞΑΝΔΡΟΣ Ο ΔΙΟΣ ΤΟΝ ΑΙΑΝΤΑ ΤΩΙ ΗΛΙΩΙ). Alexander did not continue, thus leaving all the headwaters of the Indus River unconquered. He later founded Alexandria Nikaia (Victory), located at the battle site, to commemorate his triumph. He also founded Alexandria Bucephalus on the opposite bank of the river in memory of his much-cherished horse, Bucephalus, who had carried Alexander through the campaign on the Indian subcontinent and had died heroically during that  battle at Hydaspes. Catheans
After crossing the River Chenab, Alexander joined by Porus with elephants and 5,000 local troops, laid siege to Sagala, where the Cathaeans (related to Kāṭhī) had entrenched themselves. The city was razed to the ground, and many of its inhabitants killed:
"The Cathaeans... had a strong city near which they proposed to make their stand, named Sagala. (...) The next day Alexander rested his troops, and on the third advanced on Sangala, where the Cathaeans and their neighbours who had joined them were drawn up in front of the city. (...) At this point too, Porus arrived, bringing with him the rest of the elephants and some five thousand of his troops. (...) Alexander returned to Sangala, razed the city to the ground, and annexed its territory". Arrian, Anabasis of Alexander, V.22-24

Musicanus
Musicanus (, Indian: Mûshika) was an Indian king at the head of the Indus, who raised a rebellion against Alexander the Great . Peithon, one of Alexander's generals, managed to put down the revolt:

"Meantime he was informed that Musicanus had revolted. He dispatched the viceroy, Peithon, son of Agenor, with a sufficient army against him, while he himself marched against the cities which had been put under the rule of Musicanus. Some of these he razed to the ground, reducing the inhabitants to slavery; and into others he introduced garrisons and fortified the citadels. After accomplishing this, he returned to the camp and fleet. By this time Musicanus had been captured by Peithon, who was bringing him to Alexander." - Arrian Anabasis

Porticanus

The King of Patala, Porticanus came to Alexander and surrendered. Alexander let him keep possession of his own dominions, with instructions to provide whatever was needed for the reception of the army. Sambus
Sambus was yet another ruler in lower Indus valley. According to Diodorus Siculus, Alexander invaded his kingdom, killing eighty thousand people and destroying cities. Most of population was taken into slavery, and Sambus himself ‘fled with thirty elephants into the country beyond the Indus’. Revolt of Alexander's army

East of Porus's kingdom, near the Ganges River (the Hellenic version of the Indian name Ganga), was the powerful Nanda Empire of Magadha and the Gangaridai Empire of Bengal. Fearing the prospects of facing other powerful Indian armies and exhausted by years of campaigning, his army mutinied at the Hyphasis River (the modern Beas River), refusing to march further east. Alexander, using the incorrect maps of the Greeks, thought that the world ended a mere  away, at the edge of India. He therefore spoke to his army and tried to persuade them to march further into India, but Coenus pleaded with him to change his mind and return, saying the men "longed to again see their parents, their wives and children, their homeland". Alexander, seeing the unwillingness of his men, agreed and turned back. Mallian campaign

Along the way, his army conquered the Malli clans (in modern-day Multan). During a siege, Alexander jumped into the fortified city with only two of his bodyguards and was wounded seriously by a Mallian arrow. His forces, believing their king dead, took the citadel and unleashed their fury on the Malli who had taken refuge within it, perpetrating a massacre, sparing no man, woman or child. However, due to the efforts of his surgeon, Kritodemos of Kos, Alexander survived the injury. Following this, the surviving Malli surrendered to Alexander's forces, thereby his beleaguered army moved on and conquered more Indian tribes along the way. Aftermath

Alexander sent much of his army to Carmania (modern southern Iran) with his general Craterus, and commissioned a fleet to explore the Persian Gulf shore under his admiral Nearchus while he led the rest of his forces back to Persia by the southern route through the Gedrosian Desert (now part of southern Iran) and Makran (now part of Pakistan). In crossing the desert, Alexander's army took enormous casualties from hunger and thirst, but fought no human enemy. They encountered the "Fish Eaters", or Ichthyophagi, primitive people who lived on the Makran coast of the Arabian Sea, who had matted hair, no fire, no metal, no clothes, lived in huts made of whale bones, and ate raw seafood obtained by beachcombing. During the crossing, Alexander refused as much water as possible, to share the sufferings of his men and to boost the morale of the army. In the territory of the Indus, Alexander nominated his officer Peithon as a satrap, a position he would hold for the next ten years until 316 BC, and in the Punjab he left Eudemus in charge of the army, at the side of the satrap Porus and Taxiles. Eudemus became ruler of a part of the Punjab after their death. Both rulers returned to the West in 316 BC with their armies. In BC, Chandragupta Maurya of Magadha founded the Maurya Empire in India and conquered the Macedonian satrapies during the Seleucid–Mauryan war (305–303BC). 