Little is known of his reign until 208 BC when he was attacked by Antiochus III the Great, whom he tried in vain to resist on the shores of the river Arius, the modern Herirud. The war lasted three years, and Euthydemus was besieged in his capital Bactra (modern Balkh). But Antiochus saw that he was not able to subdue Bactria and Sogdiana, and so in 206 concluded a peace with Euthydemus, in which he recognized his legitimacy, also promising to betroth one of his daughters to Euthydemus' son Demetrius.
There exist many coins of Euthydemus, portraying him as a young, middle-aged and old man. Some of them could be memorative issues by his son and a later king, Agathocles. The attribution of some of his coins (the younger portraits) to a second Euthydemus has been suggested but the evidence is very vague. The end of his reign can not be fixed, but he was succeeded by Demetrius, who went on to conquer northern India.
And these were the persons whom I showed to Euthydemus, telling him that they were all eager to learn: to which Ctesippus and all of them with one voice vehemently assented, and bid him exhibit the power of his wisdom.
Euthydemus, observing this, determined to persevere with the youth; and in order to heighten the effect went on asking another similar question, which might be compared to the double turn of an expert dancer.
They are not "in pari materia," Euthydemus, said Ctesippus, and you had better take care, for it is monstrous to suppose that your father is the father of all.
And these were the persons whom I showed to Euthydemus, telling him that they were all eager to learn : to which Ctesippus and all of them with one voice vehemently assented, and bid him exhibit the power of his wisdom.
Euthydemus, observing this, determined to persevere with the youth ; and in order to heighten the effect went on asking another similar question, which might be compared to the double turn of an expert dancer.
I am inclined to think, however, Dionysodorus and Euthydemus, that this argument lies where it was and is not very likely to advance : even your skill in the subtleties of logic, which is really amazing, has not found out the way of throwing another and not falling yourself, now any more than of old.