In B.C. 280 Seleucus was assassinated by one of his officers, and was succeeded by Antiochus I. In B.C. 256, under the rule of Antiochus II., Diodotus, known as “Governor of the thousand cities of Bactria,” threw off his allegiance and assumed sovereignty, thus founding the Græco-Bactrian kingdom.[18] Polybius[19] tells us that Diodotus was superseded by Euthydemus, who was in the enjoyment of power at the time of Antiochus the Great’s expedition to the East--about B.C. 208. Euthydemus was defeated by Antiochus, but appealed to his victor’s generosity, and pointed out the grave danger that would arise if he were obliged to call in the aid of the Scythians, who were already hovering on the Chinese frontier of his dominions.[20] Antiochus finally agreed to acknowledge his independence. In B.C. 250 a certain Arsaces, who seems by his coins to have been the chief of a band of Dahæ Scythians dwelling near the Oxus, overthrew Andragoras, nominally satrap of Parthia, and set himself up as king of Parthia.[21] He was the founder of the famous dynasty of the Arsacidæ. As Mr. Gardner[22] observes, the “so-called history of Parthia is really the history of Central Asia under the Arsacidæ.” After a reign of two years he was killed in battle, leaving his kingdom to his brother Tiridates, who was the real founder of the Parthian power. The fifth king of this dynasty was Mithridates (B.C. 190), who extended his conquests to such a degree that, according to Justin, his sway included the Himalayas and the Euphrates.[23] He also compelled Eucratides, the powerful king of Bactria, who had come to the throne about B.C. 170, to cede certain districts of his kingdom. After a glorious reign he died about B.C. 140, and was succeeded by his brother Phraates.[24] The Syrian Empire of the Seleucidæ was fast falling to pieces, and Parthia was never again invaded by the Greeks. But a more terrible foe was approaching from the East,[25] for it now came into collision with a Scythian band, called “Su” or “Se” in the Chinese annals, which in the second century B.C. had overrun the provinces bordering the Jaxartes. They are identical with the Sacæ of classical writers, and were afterwards known in Upper India as the Sakas. Phraates[26] summoned a band of these savages to aid him against the Syrian Antiochus. Arriving at the scene of action too late to be of service in the campaign, they turned against him, defeated his army and slew him.