Demetrius I Anicetus (Dēmētrios Anikētos, "the unconquered"), also called Damaytra was a Greco-Bactrian and later Indo-Greek king (Yona in Pali language, "Yavana" in Sanskrit) (reigned c. 200–167 BC), who ruled areas from Bactria to ancient northwestern India. He was the son of the Greco-Bactrian Kingdom's ruler Euthydemus I and succeeded him around 200 BC, after which he conquered extensive areas in what is now southern Afghanistan, Iran and Pakistan and India.
He was never defeated in battle and was posthumously referred to as "the Unconquered" (Aniketos) on the pedigree coins of his successor Agathocles. Demetrius I may have been the initiator of the Yavana era, starting in 186–185 BC, which was used for several centuries thereafter.
"Demetrius" was the name of at least two and probably three Bactrian kings. The much debated Demetrius II was a possible relative, whereas Demetrius III (100 BC), is known only from numismatic evidence.
The father of Demetrius, Euthydemus, was attacked by the Seleucid ruler Antiochus III around 210 BC. Although he commanded 10,000 horsemen, Euthydemus initially lost a battle on the Arius and had to retreat. He then successfully resisted a three-year siege in the fortified city of Bactra, before Antiochus finally decided to recognize the new ruler.
The final negotiations were made between Antiochus III and Demetrius. Antiochus III was reportedly highly impressed by the demeanour of the young prince, and offered him one of his daughters in marriage, around 206 BC:
"And after several journeys of Teleas to and fro between the two, Euthydemus at last sent his son Demetrius to confirm the terms of the treaty. Antiochus received the young prince; and judging from his appearance, conversation, and the dignity of his manners that he was worthy of royal power, he first promised to give him one of his own daughters, and secondly conceded the royal title to his father." Polybius 11.34
The term used for "young prince" is neaniskos (νεανίσκος), suggesting an age around 16, which in turn gives a birth date for Demetrius around 222 BC.