The births of Mahavira and Buddha in the 6th century BC mark the beginning of well-recorded history in the region. Around the 5th century BC, the ancient region of Afghanistan and Pakistan was invaded by the Achaemenid Empire under Darius in 522 BC forming the easternmost satraps of the Persian Empire. The provinces of Sindh and Panjab were said to be the richest "satraps" of the Persian Empire and contributed many soldiers to various Persian expeditions. It is known that an "Indian" contingent fought in Xerxes' army on his expedition to Greece. Herodotus mentions that the Indus satrapy supplied cavalry and chariots to the Persian army. He also mentions that the Indus people were clad in armaments made of cotton, carried bows and arrows of cane covered with iron. Herodotus states that in 517 BC Darius sent an expedition under Scylax to explore the Indus. Under Persian rule, much irrigation and commerce flourished within the vast territory of the empire. The Persian empire was followed by the invasion of the Greeks under Alexander's army. Since Alexander was determined to reach the easternmost limits of the Persian Empire he could not resist the temptation to conquer India (i.e. the Punjab region), which at this time was parcelled out into small chieftain-ships, who were feudatories of the Persian Empire. Alexander amalgamated the region into the expanding Hellenic empire. The "Rigveda", in Sanskrit, goes back to about 1500 BC. The Indian literary tradition has an oral history reaching down into the Vedic period of the later 2nd millennium BC."Ancient India" is usually taken to refer to the "golden age" of classical Indian culture, as reflected in Sanskrit literature, beginning around 500 BC with the sixteen monarchies and 'republics' known as the Mahajanapadas, stretched across the Indo-Gangetic plains from modern-day Afghanistan to Bangladesh. The largest of these nations were Magadha, Kosala, Kuru and Gandhara. Notably, the great epics of Ramayana and Mahabharata are rooted in this classical period.