Bactria (Bactriana) was the ancient Greek name of the country between the range of the Hindu Kush (Paropamisus) and the Amu Darya (Oxus), with the capital Bactra (now Balkh). It was known to the east in the Indian subcontinent as Gandhara.
It is a mountainous country with a moderate climate. Water is abundant and the land is very fertile. Bactria was the home of one of the Iranian tribes. Modern authors have often used the name in a wider sense, as the designation of the whole eastern part of Iran. As there can be scarcely any doubt that it was in these regions, where the fertile soil of the mountainous country is everywhere surrounded and limited by the Turanian desert, that the prophet Zoroaster preached and gained his first adherents, and that his religion spread from here over the western parts of Iran, the sacred language in which the Avesta, the holy book of Zoroastrianism, is written, has often been called "old Bactrian." But there is no reason for this extensive use of the name, and the term "old Bactrian" is, therefore, at present completely abandoned by scholars.
Whether Bactria formed part of the Median empire, we do not know; but it was subjugated by Cyrus and from then formed one of the satrapies of the Persian empire. When Alexander had defeated Darius III, his murderer Bessus, the satrap of Bactria, tried to organize a national resistance in the east. But Bactria was conquered by Alexander without much difficulty; it was only farther in the north, beyond the Oxus, in Sogdiana, that he met with strong resistance. Bactria became a province of the Macedonian empire, and soon came under the rule of Seleucus, king of Asia.
The Macedonians (and especially Seleucus I and his son Antiochus I) founded a great many Greek towns in eastern Iran, and the Greek language became for some time dominant there. The paradox that Greek presence was more prominent in Bactria than in areas far more adjacent to Greece could possibly be explained by the supposed policy of Persian kings to deport unreliable Greek as colonists to this the most remote province of their huge empire. The many difficulties against which the Seleucid kings had to fight and the attacks of Ptolemy II, gave to Diodotus, satrap of Bactria, the opportunity of making himself independent (about 255 BC) and of conquering Sogdiana. He was the founder of the Graeco-Bactrian kingdom. Diodotus and his successors were able to maintain themselves against the attacks of the Seleucids; and when Antiochus III the Great, had been defeated by the Romans (190 BC), the Bactrian king Euthydemus and his son Demetrius crossed the Hindu Kush and began the conquest of eastern Iran and the Indus valley. For a short time they wielded great power; a great Greek empire seemed to have arisen far in the East. But this empire was torn by internal dissensions and continual usurpations. When Demetrius advanced far into India one of his generals, Eucratides, made himself king of Bactria, and soon in every province there arose new usurpers, who proclaimed themselves kings and fought one against the other.
Most of them we know only by their coins, a great many of which are found in Afghanistan and India. By these wars the dominant position of the Greeks was undermined even more quickly than would otherwise have been the case. After Demetrius and Eucratides, the kings abandoned the Attic standard of coinage and introduced a native standard, no doubt to gain support from outside the Greek minority. In India, the syncretism went even further. King Milinda/Menander of India, known as a great conqueror, even converted to Buddhism. His successors managed to cling to power somewhat longer, but around 10 AD all Greek kings were gone.
The weakness of the Graeco-Bactrian empire was shown by its sudden and complete overthrow, but then its emergence, isolated thousands of miles from Greece, could only be described as a paradox. However, its cultural influences were not completely undone; an artistic style mixing western and eastern elements known as the Gandhara culture survived the empire for hundreds of years.
Initial text From a 1911 encyclopedia. Please update as needed.