The lower basin of the Indus forms a natural boundary between the Iranian Plateau and the Indian subcontinent; this region embraces all or parts of the Pakistani provinces Balochistan, Khyber Pakhtunkhwa, Punjab and Sindh and the countries Afghanistan and India. The first West Eurasian empire to annex the Indus Valley was the Persian Empire, during the reign of Darius the Great. During his reign, the Greek explorer Scylax of Caryanda was commissioned to explore the course of the Indus. It was crossed by the invading armies of Alexander, but after his Macedonians conquered the west bank—joining it to the Hellenic world, they elected to retreat along the southern course of the river, ending Alexander's Asian campaign. Alexander's admiral Nearchus set out from the Indus Delta to explore the Persian Gulf, until reaching the Tigris River. The Indus Valley were later dominated by the Mauryan and Kushan Empires, Indo-Greek Kingdoms, Indo-Scythians and Hepthalites. Over several centuries Muslim armies of Muhammad bin Qasim, Mahmud of Ghazni, Mohammed Ghori, Tamerlane and Babur crossed the river to invade Sindh and Punjab, providing a gateway to the Indian subcontinent.