India, in Alexander's time, meant the land of the Indus–not necessarily the area where the modern country of India stands. The Greeks, who had limited knowledge of the geography of central Asia, knew almost nothing of the Indian subcontinent or China. India, to the Greeks, meant the area in western Pakistan, particularly the Punjab and Sind territories.
There are several possible reasons why Alexander chose to pursue India. Part may be simply that Persia had once possessed parts of India, and therefore Alexander, as the new Great King, wanted to reclaim it. As little was known about India, curiosity was likely also a factor. Perhaps most important, India was the end of Asia as far as Alexander knew; its acquisition was necessary if he was to rule the entire continent.
The invasion of India began in the summer of 327 B.C. Alexander proceeded as he had in his Persian conquest, vanquishing city by city. Many cities surrendered without a fight; those that did not were usually massacred without mercy. Alexander soon gained the support of Ambhi, the ruler of Attock. Alexander and his troops rested for a couple of months in the capital city of Taxiles as they prepared to meet Ambhi's enemy, Porus.
In response to Alexander's request that he submit, Porus assembled his army and prepared to meet Alexander on the bank of the Hydaspes River. When Alexander arrived, he found that Porus had the fords guarded with elephants, which made a crossing impossible. Moreover, whenever Alexander moved along the river, Porus mirrored him on the opposite side. To confuse his foe, Alexander divided his army into several units and spread them along the bank. This splitting up also gave Alexander a chance to search for other possible fords farther down; indeed, a suitable one was found seventeen miles upstream. The question was whether Alexander could keep Porus from following him all the way to that crossing point.
Once again Alexander devised a plan to confuse his enemy. For several nights, he sent the cavalry to various spots along the bank and instructed them to make noise and raise war cries. Porus, of course, followed them the first few times, but eventually stopped responding to Alexander's bluffs. On the night planned for the attack, Alexander divided the troops into three groups. One would remain in the original spot to keep Porus off guard, while a second group prepared for a crossing that would take place only if Alexander succeeded in clearing the fords. Alexander himself led the third group, consisting of about 15,000 infantry and 5,500 cavalry. Porus sent an initial group of about 2,000 cavalry, led by his son, to attack the Macedonians while they were crossing and to drive them back into the river. However, the Indians did not make it in time to have the early advantage, and Alexander easily defeated the troops.
Porus was therefore forced to march against Alexander with full force, leaving only a small detachment to face the second crossing group. The fact that Porus's front line consisted entirely of elephants prevented Alexander from using his cavalry, as the horses would not charge in face of the elephants. Once again, Alexander succeeded with a brilliant strategy. He kept a segment of his cavalry hidden, allowing Porus to think that he was winning. When Porus advanced to exploit Alexander's apparent weakness, the hidden cavalry emerged and caused confusion among the already exposed Indians. The battle culminated in the surrounding of the Indians, and Porus was finally prevailed upon to surrender. The victory had not been easy, however. The Macedonians were particularly troubled about the elephants, which had brutally trampled and mangled their soldiers. Nevertheless, it was Alexander's last major battle and one of his greatest.
Alexander allowed Porus to continue his rule–a decision likely motivated by Alexander's recognition that he was running out of resources to maintain a strong presence at every corner of his territory. Nevertheless, Alexander's thirst was not quenched, and he wanted to press farther, though his next opponent, the Nanda empire, would have been very formidable. Alexander's troops had other plans, however, and talks of mutiny abounded. The troops had been away for eight years and marched over 17,000 miles. The elephants had been especially demoralizing, especially since it was reported that Nanda possessed about 4,000 of them. Alexander offered every possible incentive and bribe, but even his chief officers sympathized with the men. One senior officer, Coenus, finally rose to speak on behalf of the men, and Alexander finally recognized that a rebellion led by a popular man like Coenus was an alarming possibility.
Alexander, therefore, he was finally prevailed upon to turn around and head home, though he never forgave his men and officers. He was convinced that he could have conquered the entire world if his men had not turned their backs on him. Furthermore, he showed no apparent gratitude for their service and dedication. He purposely took a difficult journey home that required constant skirmishes with unconquered Indian provinces. Alexander's armies finally left India by sea in September 325 B.C.
Some sources have exaggerated Alexander's success, particularly in his domain over India. In reality, Alexander's influence in the area was limited. Porus was essentially an independent ruler, though formally he derived power from Alexander. Moreover, Alexander did not have the resources to hold India in line, and by 317 B.C. all traces of Macedonian power had essentially disappeared. Nevertheless, Alexander had led a great expedition to unfamiliar territory, and he had conquered it as effectively as he had conquered the rest of Asia.