Hinduism and Transition [600 BC - 322 BC]
Hinduism had become much more than a religion; it was a way of life. Caste was the social manifestation of the underlying Hindu concept of reincarnation, causality and duty. The four original castes split up over the ages into myriads of subcastes. There was an elaborate system of precedence by which each group had a place above and below other groups. This was accepted due to the belief in reincarnation by which one was born into a particular group as a result of the reward or punishment for one's action in a previous life. Brahmins dominated over other the castes. The way of life of individuals was based on their birth and there was increase in ritualism and sacrifice.
But, there were rebels against ritual, sacrifice
and above all, against caste. Hinduism saw it's first rebel in Mahavira who
founded Jainism and Gautama Buddha who founded Buddhism.
Buddhism disappeared from India by the 12th century AD. Perhaps the Buddha's ideas were taken over by the great Hindu philosopher Shankaracharya in the ninth century AD, and were readapted to Hinduism, so as to make Buddhism no longer necessary. Whatever the reason, the Buddha who had preached the absence of God became a Hindu god!
The dominant philosophical outlook which the acharya created has persisted in Hinduism till today. During the next seven centuries, Ramanujacharya, Madhwacharya and Valabacharya preached Hinduism in the light of their own individual philosophical conception. They carried the torch of Hinduism through those centuries and made it continual vital force of this day. As iron implements helped clear the dense forests of the Gangetic plains, civilization expanded eastwards. The new agricultural tools and implements improved the knowledge of cultivation. Gradually, 16 larger territorial states (Mahajanapadas) were formed. Of these, Magadha, Kosala, Vatsa and Avanti were powerful. They fought amongst themselves for political pre-eminence for about a hundred years.
Magadha, under the leadership of Bimbisara (542 BC - 493 BC) and Ajatshatru (493 BC - 461 BC) emerged victorious. The victory of Magadha was a victory for the monarchical system, which was now firmly established in the Ganges plain. Ajat Shatru was succeeded by Udayin (460 BC - 444 BC), whose reign saw the raising of a new capital at Pataliputra (modern day Patna). The architecture of the Magadhan empire is the first of which we have any contemporary record.
The Shishunaga dynasty, which followed in 413 BC, lasted barely half a century and gave way to the Nanda dynasty. The Nandas, who had a vast standing army are sometimes described as the first empire builders of India.
In 516 BC, the Persian emperor Darius annexed Punjab and for many years, the Indian satrap continued to pay a huge tribute to the Achaemenian king. Alexander, the king of Macedonia, crossed the Hindu Kush after subduing the Achaemenians, pursuing his dreams of a world conquest.