By the sixth century B.C., knowledge of Indian history becomes more focused because of the available Buddhist and Jain sources of a later period. Northern India was populated by a number of small princely states that rose and fell in the sixth century B.C. In this milieu, a phenomenon arose that affected the history of the region for several centuries--Buddhism. Siddhartha Gautama, the Buddha, the "Enlightened One" (ca. 563-483 B.C.), was born in the Ganges Valley. His teachings were spread in all directions by monks, missionaries, and merchants. The Buddha's teachings proved enormously popular when considered against the more obscure and highly complicated rituals and philosophy of Vedic Hinduism. The original doctrines of the Buddha also constituted a protest against the inequities of the caste system, attracting large numbers of followers.

Until the entry of the Europeans by sea in the late fifteenth century, and with the exception of the Arab conquests of Muhammad bin Qasim in the early eighth century, the route taken by peoples who migrated to India has been through the mountain passes, most notably the Khyber Pass, in northwestern Pakistan. Although unrecorded migrations may have taken place earlier, it is certain that migrations increased in the second millennium B.C. The records of these people -- who spoke an Indo-European language -- are literary, not archaeological, and were preserved in the Vedas, collections of orally transmitted hymns. In the greatest of these, the "Rig Veda," the Aryan speakers appear as a tribally organized, pastoral, and pantheistic people. The later Vedas and other Sanskritic sources, such as the Puranas (literally, "old writings" -- an encyclopedic collection of Hindu legends, myths, and genealogy), indicate an eastward movement from the Indus Valley into the Ganges Valley (called Ganga in Asia) and southward at least as far as the Vindhya Range, in central India. A social and political system evolved in which the Aryans dominated, but various indigenous peoples and ideas were accommodated and absorbed. The caste system that remained characteristic of Hinduism also evolved. One theory is that the three highest castes -- Brahmins, Kshatriyas, and Vaishyas -- were composed of Aryans, while a lower caste -- the Sudras -- came from the indigenous peoples.

At about the same time, the semi-independent kingdom of Gandhara, roughly located in northern Pakistan and centered in the region of Peshawar, stood between the expanding kingdoms of the Ganges Valley to the east and the Achaemenid Empire of Persia to the west. Gandhara probably came under the influence of Persia during the reign of Cyrus the Great (559-530 B.C.). The Persian Empire fell to Alexander the Great in 330 B.C., and he continued his march eastward through Afghanistan and into India. Alexander defeated Porus, the Gandharan ruler of Taxila, in 326 B.C. and marched on to the Ravi River before turning back. The return march through Sindh and Balochistan ended with Alexander's death at Babylon in 323 B.C.