During the Alexander’s period, the western borderland of India was occupied by two Maha-Janpadas, namely Kamboja and Gandhara, which included the region of modern Punjab, Sindh, and Afghanistan.
In 522-486 B.C. (during the reign of Darius), Achaemenians extended their empire to the northwestern part of India and they had subjugated a number of principalities living to the south of the Hindukush Mountains.
Herodotus, the famous Greek historian, mentions that Darius sent a naval expedition in 517 B.C. to explore the valley of the Sindhu River.
The evidences prove that the Indian soldiers were the part of the Achaemenian army that conquered Greece in the time of Xerxes (486-465 B.C.) and also fought against Alexander at Gaugamela in 330 B.C.
The defeat of Darius III (the Achaemenian king) by Alexander became a turning point. Alexander dismantled the Persian Empire and captured most of the western Asia including Iraq and Iran.
In 326 B.C., after the conquest of the Persian Empire, Alexander marched to India through the Khyber Pass.
It is surprising that no Indian source mentions anything about Alexander or his campaign. The history of Alexander's campaign in India has been reconstructed on the basis of accounts available in Greek and Roman sources. It is also surprising to note that Greek sources are completely silent about Kautilya.
The Greek sources, however, mention about Sandrocottas or Androcottas who has been identified as Chandragupta Maurya and fixing 326 B.C. as the date of Chandragupta's accession to the throne.
In 326 B.C. when Alexander reached on the Indian soil, the king of Takshasila near Rawalpindi in Punjab offered help to him. But many of the Republican Chiefs and kings in Afghanistan, Punjab, and Sindh had poised a brave resistance and refused to submit to the Alexander without a fight.
Alexander divided his army into two parts after crossing Hindukush and Alexander himself, conquers the north-western part of India.
The Greeks had faced a strong resistance from tribal Hasti chief having capital Pushkalavati.
The army of Assakenoi king was led by the queen, which was the example of an enthusiasm for the defense of the country by the people of these regions that even women and the mercenaries took part in fighting and preferred a glorious death.
In spite of a hard resistance for many days (by the Assakenoi soldiers), Alexander captured the city Massaga (the capital of Assakenoi).
After the victory of Assakenoi, Alexander had himself resolved a special agreement by which he granted the lives of the army of 7,000 mercenaries. But deceitfully, they had been slaughtered mercilessly in the night by Alexander and his soldiers. This massacre of Assakenoi has been condemned even by the Greek writers.
Alexander, after defeating Assakenoi, joined his other division of army and constructed a bridge on the Indus River nearby Attock.
After crossing the Indus, Alexander proceeded towards Taxila, but the king Ambhi admited the sovereignty of the Alexander.
Paurava (Greeks call Porus), the ruler of a kingdom between the Jhelum and the Chenab was the most powerful among the north-western Indian provinces. Alexander made intense preparations to defeat him.
Porus fought bravely and with nine wounds on his body, was led a captive before Alexander.
When Porus was brought as captive before Alexander, he (Alexander) asked him how he would like to be treated. Porus replied proudly, "Like a King".
Alexander made a coalition with the brave king Porus by restoring his kingdom and adding to it the territories of 15 republican states along with 5,000 cities and villages.
Alexander had to fight hard with the Kathaioi (Kathas) on the bank of river Beas. The casualties tolled up to 17,000 killed and 70,000 captured.
In July 326 B.C., on the bank of the Beas, the Alexander’s campaign was clogged due to soldiers’ mutiny and they refused to proceed further.
The Greek writers recorded the reason of the disobedience of the soldiers was merely the war weariness or partly to the fear inspired by the mighty empire of the Nandas.
The troops were worried, if Alexander met with an accident in course of the campaign what would be the fate of the whole army. They were also afraid of the other unpredictable calamity. Hence, due to refusal of the army, Alexander decided to return.
Alexander had to fight with a confederacy of republican states led by the Malloi (Malavas) and the Oxydrakai (Kshudrakas) near the confluence of the Jhelum and the Chenab Rivers.
About 5,000 Brahmans gave up their pen for the sword to save their mother land. All the towns of the Malavas became citadels of resistance.
Alexander was severely wounded while capturing the town and on this, his soldiers became infuriated and started killing everybody they found, irrespective of age and sex.
A tribal group, Agalassoi (Arjunayanas), also fought with great courage and showed great bravery, patriotism, and sacrifice when one of their towns was captured by Alexander. All the citizens 20,000 in number threw themselves into the fire with their wives and children.
Alexander reached Patala and began his homeward journey in September 325 B. C. He proceeded with his army by land, but sent the ships under Nearchus (one of the officers).
In 324 B.C., Alexander reached Susa in Persia where he died in the next year i.e. 323 BC.
After Alexander’s death, the Greek edifice collapsed within a short period of time.
The Greek historians had been thrilled by such victorious campaign and recorded in minute details of the impact of Alexander's Campaign.
Whereas the Indian sources have remained silent over such a triumphant Campaign because this Campaign touched only the western border of the then India and returned without leaving any lasting impact on Indian people.
In an Indian context, Alexander's campaign can hardly be called a great military success, as they showed barbarism in the conquest of petty (small) states.
The conquered region (of Alexander) declared their independence within three months of Alexander's departure from India.