The campaign was a resounding success, but Tiberius could not subjugate the Marcomanni because he was soon summoned to the Rhine frontier to protect Rome's new conquests in Germania. He returned to Rome and was consul for a second time in 7 BC, and in 6 BC was granted tribunician power (tribunicia potestas) and control in the East, positions that Agrippa had held before him. Midlife Retirement to Rhodes In 6 BC, while on the verge of accepting command in the East and becoming the second-most powerful man in Rome, Tiberius announced his withdrawal from politics and retired to Rhodes. The motives for Tiberius's withdrawal are unclear. Some historians have speculated that Tiberius and Drusus were only ever intended as caretakers, and would have been swept aside once Julia's two sons by Agrippa, Gaius and Lucius, were adopted as Augustus' heirs and came of age. The promiscuous, and very public behaviour of his unhappily married wife, Julia, may have also played a part. Tacitus understood this to be Tiberius' innermost reason for moving to Rhodes, a reflection of his hatred of Julia and his longing for Vipsania. Tiberius, forbidden to see the woman he loved, found himself married to a woman he loathed, and publicly humiliated by her nighttime escapades in the Roman Forum. Whatever Tiberius' motives, his withdrawal was almost disastrous for Augustus' succession plans. Gaius and Lucius were still in their early teens, and Augustus, now 57 years old, had no immediate successor. There was no longer a guarantee of a peaceful transfer of power after Augustus' death, nor a guarantee that his family, and therefore his family's allies, would continue to hold power should the position of Princeps survive. Somewhat melodramatic stories tell of Augustus pleading with Tiberius to stay, even going so far as to stage a serious illness. Tiberius' response was to anchor off the shore of Ostia until word came that Augustus had survived, then sailing straightway for Rhodes. Tiberius reportedly regretted his departure and requested to return to Rome several times, but each time Augustus refused his requests. Heir to Augustus With Tiberius' departure, succession rested solely on Augustus' two young grandsons, Lucius and Gaius Caesar. The situation became more precarious in AD 2 with the death of Lucius. Augustus, with perhaps some pressure from Livia, allowed Tiberius to return to Rome as a private citizen and nothing more. In AD 4, Gaius