After Bimbisara his son Ajatashatru became the ruler of Magadha. Ajatashatru was also called kunika. His conquests further contributed to the growth of Magadh as an imperial power. During the reign of his father, he was the governor of Anga. According to different scholars, he reigned from 551 to 519 B.C.

Expansion of empire
Like his father, Ajatashatru followed the imperialistic policy. Haryanak dynasty reached the climax of its glory during the reign of Ajata shatru.

War with Kosala
Being infuriated due to the murder of Bimbisara and also for considering Ajatsatru indirectly responsible for the death of Kosaladevi, who was his sister, Prasenjit stopped the annual revenue of Kasi. This angered Ajata shatru and there ensued a war between Kosala and Magadha. As remarked by Dr. Radha Kumud Mookerji, "But his first campaign against Kosala was a failure. Entrapped in an ambush he had to surrender to the king of Kosala with his whole army. Prasenjit, however, as a pacific Buddhist, ended his conflict with Ajatsatru by giving him back his liberty, army, kingdom, and additionally a daughter in marriage named Vajira."

War with Lichchhavis of Vaishali
The second campaign of Ajatsatru was against the Lichchhavis of Vaishali. Bimbisara had established matrimonial alliance with Vaishali by marrying Chellana, the sister of king Chetak. According to Jain literature Halla and Vchalla were the two sons of Bimbisara. They were born of the Lichchavi princess. Bimbisara had given them art elephant named Seyanaga and necklace of 18 strings as gifts, which they carried to Vaishali. Ajatsatru asked these gifts back but Chetak refused. Being enraged Ajatsatru declared war against Chetaka.

However, according to Buddhist literature near river Ganges that flowed between Magadha and Vijji, there was a jewel mint in which both Magadha and Lichchhavis had equal share of jewels. On being refused to get his share of the jewels, Ajatsatru declared war against the Lichchhavis. He was conscious of the power of the Lichchhavis and therefore he first sent ministers, sunidha and saskara to sow these seeds of discord, disunion and dissension among the Lichchhavis. So when he attacked, Lichchhavis could not unite to face the attack and ultimately he conquered Vaishali. Dr. Ray chaudhari has remarked, "The Kosalan war and the Vijjian war were probably not isolated events but parts of a common movement directed against the establishment of the hegemony of Magadha. The flames fused together into one big conflagration."

Religion of Ajatashatru
There is a great controversy regarding the religion of Ajatsatru. According to Dr. Radha Kumud Mookerji, "It is difficult to say whether Ajatsatru was a Jain or Buddhist. The texts of both the religions describe him a devotee of each. Jain literature does not mention him as a murderer of his father. According to it Bimbisara had committed suicide. Moreover, according to Jain literature he along with his wife went to Vaisali to meet Mahavira and praised the works of Jain monks, lastly had he been a Buddhist, there was no reason of his being jealous of Buddha. On the other hand certain other scholars say that Ajatsatru was the follower of Buddhism. According to `Mahaparinirvana sutta`, he built a stupa at Rajagriha on the relics of Buddha. Secondly, for the first Buddhist council, he built an assembly house in the rock cave of Vebhara. The sculpture of bharhut of about 2nd century B.C. also indicates that he was a Buddhist. There are certain scholars who say that in the beginning he was Jain but after the death of his father, he become a Buddhist and had confessed his guilt before lord Buddha and asked for his pardon. In the opinion of Dr. V. Smith, "Probably Ajatsatru like many later Indian sovereigns, did not confine his royal favour to any one sect, but at different times patronized followers of `former Buddha` led by Devadata, the adherent of reformed Buddhism and the Jains."