The political fight among these mahajanapadas led ultimately to one of them namely Magadh to emerge as the most powerful state and the centre of a vast empire. The earliest important ruler of Magadh was the king Bimbisara, who ruled for 52 years from 544 BC to 492 BC.
He pursued a three-pronged policy, namely, matrimonial alliances, friendship with strong rulers and conquest of weak neighbours to expand the empire.
Under the policy of matrimonial alliances, he married the sister of Prasenjit, the king of Kosala. She brought in dowry the territory of Kashi, which yielded a revenue of 1,00,000 coins. The control over Kasi and friendship with Prasenajit allowed Magadh to concentrate on other areas.
His other wives were daughters of the chiefs of Lichchavi and Madra (middle Punjab) respectively. He also conquered Anga by defeating its ruler Brahmadatta. Anga and specially its capital Champa were important for the inland and maritime trade. Thus, Kashi and conquest of Anga became the launching pad for the expansion of Magadh. He was a contemporary of both Buddha and Mahavira and paid equal respect to them.
It seems that he was either killed or forced to commit suicide by his son Ajatasatru, who was eager to take over the throne himself. Ajatasatru was an aggressive person and first came into conflict with his maternal uncle Prasenajit, who was aggrieved by the treatment meted out to Bimbisara. He asked Ajatasatru to return the territory of Kasi, which was given to his mother in dowry. Ajatasatru refused and it was only after a fierce battle Prasenajit agreed to leave Kasi with Magadh.
Similarly he fought with his maternal grandfather Chetak, the chief of Vaishali and after 16 long years of war Ajatasatru succeeded in breaking the might of Vaishali. Therefore, he not only retained Kasi, but also added Vaishali to Magadh.
Ajatasatru was succeeded by Udayin and his main contribution was building a fort on the confluence of river Ganga and river Son at Pataliputra or Patna. It was strategically a significant step as this site was not only centrally located but also allowed easy movement of merchant and soldiers.
Udayin was succeeded by the dynasty of Shishunaga. The most important achievement of Shishunaga was to defeat Avanti (Malwa) and make it a part of Magadh. The successor of Sisunaga was his son Kalashoka. It was during his rule the second Buddhist council was held.
The Shisunaga dynasty was succeeded by the kings of the Nanda dynasty. Mahapadma Nanda was its most important ruler. According to the Brahmanical texts he belonged to a low caste or at least a non-kshatriya caste. He possessed a large army and added Kalinga to his empire.
The last Nanda king was Dhannanand. He is believed to be an arrogant and oppressive ruler who imposed heavy taxes on the common man. It made them quite unpopular among the masses and ultimately Chandragupta took advantage of this public resentment and uprooted the Nanda rule and set up the Mauryan Empire.
The question is how Magadh could establish gradually its dominance over all other states of the period. Magadh certainly benefited from numerous able and ambitious rulers, but its strength was based primarily on certain geographical factors. Its earlier capital Girivraja or Rajagir was surrounded by five hills, which helped it to provide natural fortification. Secondly, its fertile river plain provided a vast amount of agricultural surplus, which was essential for raising a vast standing army. Forests in southern areas gave it timber and elephants.
Magadh had another advantage in its control over iron deposits found very near south Bihar. Such access to iron made Magadhan weapons far superior and agriculture tools more productive. It was this material background which helped Magadh to become more powerful than other mahajanapadas.