Augustus was confronted with the possibility that he would not father the strong male heir he needed under the NO MORE WAR principle. Employing the GIRLS ARE MUMS and DADS ARE BEST principles where did he go next?
Look at the picture while you think it over.
Julius Caesar, Augustus’ adoptive father, left no children, especially after Augustus allegedly killed Caesarion, his son by Cleopatra, in 30 BC.
Augustus biological father, Gaius Octavius had one other child, Augustus’ sister, Octavia.
Octavia had a number of children, but the only boy was Marcus Claudius Marcellus, officially a member of the Claudius family, but Augustus’ nephew and the only grandson of Augustus’ dead biological father.
(The noble family of the Claudii was huge. Marcellus was not closely related to Livia’s sons.)
Yes, like his adoptive father, Julius Caesar, Augustus looked for an heir descended from his own father. But which father? Augustus did nothing to further the biological line of Julius Caesar. The father Augustus looked to was his biological father, Gaius Octavius.
So Augustus’ predictable first move (DADS ARE BEST) was a cousin marriage, bringing Marcellus into the succession by (GIRLS ARE MUMS) marrying his daughter Julia 1 to him in 25 BC. Now, when and if Julia had a son, Augustus’ grandson, the boy would be Augustus’ own father’s great grandson twice over, on both his mother’s and father’s sides; for Augustus, this was the next best thing to having a son of his own. Augustus didn’t adopt Marcellus because he hoped there was plenty of time for the grandson to turn up in. There wasn’t. Marcellus died childless in 23 BC.