Demetrius had been sent to Rome as a hostage by his father, Seleucus Philopater. Antiochus having afterwards assassinated Seleucus, and made himself King of Syria, Demetrius asked the Senate to restore him his liberty and his throne. But, according to Polybius, although the senators were touched by the words of the young prince, they thought it more to the interest of the Republic to detain him in Rome, and to recognise the son of Antiochus.

Some time after, Demetrius wished to renew his appeal to the Senate, and he consulted Polybius, who tried to dissuade him from it: “Do not,” said the historian, “bruise yourself a second time against the same stone. Believe in yourself and in yourself alone, and prove by your own boldness that you deserve to be king.”

The prince, expecting no doubt advice more in harmony with his intentions, did not follow the counsel of Polybius till he was taught the value of it by a second refusal from the Senate; and then he prepared for flight. Diodorus, who had educated him, arrived very opportunely at that moment from Syria, and assured him that if he were to present himself to his people with but one attendant at his back he would be immediately proclaimed king.

Polybius, Diodorus, and some other friends of the young prince, devoted themselves to his service. They bought a Carthaginian ship lying at the mouth of the Tiber, without much hindrance it would seem from the vigilance of the authorities; for the sale and all the arrangements, including the settlement of the very hour of departure, were effected with the utmost publicity. When the time came Demetrius assembled his friends around him, a limited number of them only being in the secret, and standing pledged to embark with their slaves at a given signal. Polybius was ill, and could not leave his house, but he became apprehensive lest the young man should abandon himself to the pleasures of the table, and forget the hour fixed for his setting out. He therefore sent a slave to him towards nightfall, with orders to approach him as though on business of importance, and to place a letter in his hand reminding him of his duty. Demetrius read the letter, invented a pretext for withdrawing from the table, and returned with his confidants to his own house, whence he sent away his servants to Anagnia with orders to get everything in readiness for a boar hunt on the next day but one—this being his favourite sport, and the one which had first brought him into contact with Polybius. His friends also gave the same orders to their slaves, and in due time all the confederates assembled at Ostia. Demetrius still pretended that he meant to stay at Rome, and that he was merely sending out some trusted friends of his own age with instructions to his brother. The captain of the ship, for his part, was not disposed to be too particular in his inquiries about anything except the money for the voyage; and towards night Demetrius and his companions quietly embarked. At daybreak the anchors were raised, the vessel stood out to sea, and the fugitives were free. (Polybius, bk. xxxi., frag. xii.)