Bactria was a historical region and satrapy in Central Asia, with Bactra serving as its capital. Located north of the Hindu Kush mountains and south of the Amu Darya river, covering the flat region in Afghanistan, Tajikistan, Uzbekistan, and northern Pakistan. In 2140 BC, the legendary Assyrian king Ninus was said to have defeated the Bactrian king Oxyartes. In the sixth century BC, the Persian Achaemenid Empire conquered Bactria from the Medes, and it became the northern province of Persia in Central Asia. In 329 BC, the Macedonian king Alexander the Great annexed Bactria, and he bequeathed Bactria to Seleucus upon his death. The Seleucids established several towns in Bactria, and the Greek language became dominant in the region for some time. In 245 BC, the Greco-Bactrian Kingdom declared its independence, but it was replaced by the Indo-Greek Kingdom in 180 BC. The region later fell to the Sassanid Empire and the White Huns, and, in the mid-7th century, the Rashidun Caliphate spread Islam to Bactria. The Bactrians became the ancestors of the modern Tajiks.