The premier State in the interior of India was once Magadha or Bihar, located in Patna and the neighbouring districts, and ruled by what was known as the Nanda line of kings. The ruler at the time of Alexander's invasion of the Punjab was said to be the son of a barber who had become a paramour of the late queen and who, with her help, murdered the king. A youth, Chandragupta by name, was believed to be an illegitimate scion of this royal house. He seems in some way to have incurred the displeasure of the king and was compelled to flee for safety to the north-west, the scene of the Macedonian triumphs.

Chandragupta visited the invaders' camp and actually met Alexander. The Nanda king of Magadha, Mahapadma by name, was unpopular, and the prospect of making an easy conquest of his kingdom was one of the reasons why Alexander was anxious to pursue his march to the further east.

It was reserved for Chandragupta, however, to expel the barber's son and to occupy his throne. With the help of a clever Brahman, Chanakya by name, as his minister, he challenged Mahapadma Nanda to battle, defeated him, and then murdered him and also his entire family. In his revolt Chandragupta had the assistance of one of the warrior Chieftains whom he had met and who at first shared his throne. But he was soon disposed of, and Chandragupta reigned alone. His rule is believed to date from about the year 321 B.C., two years after the death of Alexander. He rules for 24 years, from Pataliputra, the modern Patna, as his capital.

Chandragupta, known to the Greeks as Sandrokottos, was not content with the kingdom he had won, but took occasion by the hand to extend it to the north-west over the scene of Alexander's exploits. From the first the Indian provinces had been restive under Macedonian rule. Before Alexander had returned to Persia he heard that his Satrap, Philippus, had been murdered, and he was unable to do more than make temporary arrangements for a successor. With the help of the fierce fighting tribes of the north-west as his mercenaries, Chandragupta expelled the leaderless Macedonians and extended his own rule over north India and probably as far south as the Narmada River. Holding sway from the Bay of Bengal to the Arabian Sea, from the Himalayas to Ujjain, he became, in fact, the first Emperor of India from BIHAR.