Marcellus was plainly mortified at not being declared Augustus's heir

Marcellus was plainly mortified at not being declared Augustus's heir. He was very young, only in his twentieth year. Augustus's previous favours had given him an exaggerated sense both of his talents and of his political importance. He tried to carry the matter off by being pointedly rude to Agrippa at a public banquet. Agrippa with difficulty kept his temper; but that there was no sequel to the incident encouraged Marcellus's supporters to believe that Agrippa was afraid of him. They even told each other that if Augustus did not change his mind within a year or two Marcellus would usurp the Imperial power. They grew so rowdy and boastful, Marcellus doing little to check them, that frequent clashes occurred between them and the party of Agrippa. Agrippa was most vexed by the insolence of this young puppy, as he called him-he who had borne most of the chief offices of state and fought a number of successful campaigns. But his vexation was mixed with alarm. The impression created by these incidents was that Marcellus and he were indecently wrangling as to who should wear Augustus's signet ring after he was dead.
He was ready to make almost any sacrifice to avoid seeming to play such a part. Marcellus was the offended and Agrippa wished to put the whole burden OB him. He decided to withdraw from Rome. He went to Augustus and asked to be appointed Governor of Syria. When Augustus asked him the reason for his unexpected request he explained that he thought he could, in that capacity, drive a valuable bargain with the King of Parfhia. He could persuade the King to return the regimental Eagles and the prisoners captured from the Romans thirty years before, in exchange for the King's son whom Augustus was holding captive at Rome. He said nothing about his quarrel with Marcellus. Augustus, who had himself been greatly disturbed by it, torn between old friendship for Agrippa and indulgent paternal love for Marcellus, did not allow himself to consider how generously Agrippa was behaving, for that would have been a confession of his own weakness, and so made no reference to the matter either. He granted Agrippa’s request with alacrity, saying how important it was to get the Eagles back, and the captives-if any of them were still alive after so long-and asked how soon he would be ready to start. Agrippa was hurt, misunderstanding his manner. He thought that Augustus wanted to get rid of him, really believing that he was quarrelling with Marcellus about the succession. He thanked him for granting his request, coldly protested his loyalty and friendship, and said that he was ready to sail the following day.
He did not go to Syria. He went no farther than the island of Lesbos, sending his lieutenants ahead to administer the province for him. He knew that his stay at Lesbox would be read as a sort of banishment incurred because of Marcellus. He did not visit the province, because if he bad done so it would have given the Marcellans a handle against him: they would have said that he had gone to the East in order to gather an army together to march against Rome. But he flattered himself that Augustus would need his services before long; and fully believed that Marcellus was planning to usurp the monarchy. Lesbos was conveniently near Rome. He did not forget his commission: he opened negotiations, through intermediaries, with the King of Parthia but did not expect to conclude them for a while. It takes a deal of time and patience to drive a good bargain with an Eastern monarch.