The invasion of India by Alexander  of Macedonia in 326 BC occurred during the rule of the Nandas.  This period also witnessed the expansion and decline of the Achemenian Empire also called the Persian empire. The sequence of events in the  Persian empire was to consequently have its effect on the Indian subcontinent. Domination of the regions in the Indus valley by Dauris I making it the 20^th satrapy  of the Persian Empire contributed greatly to the Persian coffers.  After his death Xerxes  succeeded to the Indian province through his campaign against Greece. His  defeat inflicted by the Athenians  and the Spartans marked the decline of the Achaemenian empire.

Alexander was born in 356 BC. He was the successor of Philip II of Macedonia and was the groomed by Arisrotle. At the age of 20 he became the king of Macedonia owing to the murder of his father, (instigated by his mother Olympias). Having inherited a strong and unified kingdom with a strong army. He became the master of the Archeamenian empire after defeating Darius III  at the battle of Arbela or Gaugamela.

 He set out to subdue the eastern provinces of the Achalmeind. Indus was till then the  boundary of the the Persian Empire. Though he did not have any idea of conquering India, the knowledge of the natural beauty and bounty of India invoked the geographical enquiry and provoked him to venture  into India. India then was a collaboration of independent states like Taxila, Paurana and tribes such as Kshudnakas the Mallai and Abastanois dwelt on the banks of the Beas, Ravi and Chenab. Conflicts among them was the vantage point  which Alexander took advantage of. In May 327 BC Alexander crossed the Hindukush mountains. In February 326 BC he crossed the Indus and advanced to Takshasila the capital of Ambhi. He was accorded a hospitable welcome here. After this he advanced eastwards to attack Porous the king of the territory between Hydrapes (Jhelum) and Akesines (Chenab) who defied the power of Alexander. At the battle of Hydaspes Porous was defeated and was taken a prisoner. His pride and wish to be treated as a king despite his defeat won the sympathy of Alexander he was reinstat+ed to his territories and also was given more territories to extend his empire. In a short span of nineteen months Alexander paved the way  for many reforms to follow in India after his retreat in October 326 BC.

The relevance of Alexander 's invasion of India is an inseparable milestone in Indian history, as it leads to a precise measurement of the chronology of Indian history. Besides this it resulted in the establishment of Greek settlement in India. The invasion was a lesson to be learnt by the kingdom and tribes whose disunity led to their defeat. Henceforth there was a need for maintaining a greater unity in the Indian territories. The skill and discipline of the Greek army inspired the need for a better army. India thus came into contact with the European world and furthered the contacts through trade exchange of art and literature. The establishment of communication by land and sea was noteworthy. some of these were the land routes through Kabul, the Mulla pass in Baluchistan and Gedrosia. The Hellenistic influence on Indian art, expressed through the Gandhara sculptures  is a result of the Greek influence. The Greek influence restructured  the Indian state affairs and society. Alexander had made arrangements for administration of the conquered territories. After his death in June 323 BC  the territories conquered by him became independent thus leading to the disruption of the empire.