Bactria, or Bactriana, was an ancient region in Central Asia. Bactria proper was north of the Hindu Kush mountain range and south of the Oxus river, covering the flat region that straddles modern-day Afghanistan. More broadly Bactria was the area north of the Hindu Kush, west of the Pamirs and south of the Tian Shan, covering modern-day Tajikistan and Uzbekistan as well, with the Amu Darya flowing west through the center.

Called “beautiful Bactria, crowned with flags” by the Avesta, the region is one of the sixteen perfect Iranian lands that the supreme deity Ahura Mazda had created. One of the early centers of Zoroastrianism and capital of the legendary Kayanian kings of Iran, Bactria is mentioned in the Behistun Inscription of Darius the Great as one of the satrapies of the Achaemenid Empire; it was a special satrapy and was ruled by a crown prince or an intended heir. Bactria was the center of Iranian resistance against the Macedonian invaders after the fall of the Achaemenid Empire in the 4th century BC, but eventually fell to Alexander the Great. After the death of the Macedonian conqueror, Bactria was annexed by his general, Seleucus I.

Nevertheless, the Seleucids lost the region after declaration of independence by the satrap of Bactria, Diodotus I; thus started history of the Greco-Bactrian and the later Indo-Greek Kingdoms. By the 2nd century BC, Bactria was conquered by the Iranian Parthian Empire, and in the early 1st century, the Kushan Empire was formed by the Yuezhi in the Bactrian territories. Shapur I, the second Sasanian King of Kings of Iran, conquered western parts of the Kushan Empire in the 3rd century, and the Kushano-Sasanian Kingdom was formed. The Sasanians lost Bactria in the 4th century, however, it was reconquered in the 6th century. With the Muslim conquest of Iran in the 7th century, Islamization of Bactria began.

Bactria was center of an Iranian Renaissance in the 8th and 9th centuries[citation needed], and New Persian as an independent literary language first emerged in this region. The Samanid Empire was formed in Eastern Iran by the descendants of Saman Khuda, a Persian from Bactria; thus started spread of Persian language in the region and decline of Bactrian language.