Tiberius becomes colleague of Augustus

In the beginning of Church History: Tiberius becomes colleague of Augustus (A.D.12)
In 6 BC, on the verge of accepting command in the East and becoming the second most powerful man in Rome, Tiberius suddenly announced his withdrawal from politics and retired to Rhodes. The precise motives for Tiberius's withdrawal are unclear.9 Historians have speculated a connection with the fact that Augustus had adopted Julia's sons Gaius and Lucius, and seemed to be moving them along the same political path that both Tiberius and Drusus had trod. Tiberius thus seemed to be an interim solution; he would hold power only until his stepsons would come of age, and then be swept aside. The promiscuous, and very public, behavior of his unhappily married wife, Julia,10 may have also played a part;8 indeed Tacitus calls it Tiberius' intima causa, his innermost reason for departing for Rhodes, and seems to ascribe the entire move to a hatred of Julia and a longing for Vipsania.11 Tiberius had found himself married to a woman he loathed, who publicly humiliated him with nighttime escapades in the Forum, and forbidden to see the woman he had loved.
Whatever Tiberius's motives, the withdrawal was almost disastrous for Augustus's 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's 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 apocryphal stories tell of Augustus pleading with Tiberius to stay, even going so far as to stage a serious illness; Tiberius's response was to anchor off the shore of Ostia until word came that Augustus had survived, then sailing straightway for Rhodes.12 Tiberius reportedly discovered the error of his ways and requested to return to Rome several times; each time Augustus refused the request.