Before Tiberius can depart for Boiohaemum to conquer the western German Marcomanni tribe, Augustus compels him, in turn, to adopt his nineteen-year-old nephew Germanicus as a son and to name him as his heir (Tacitus, Annals IV.57).

This continues the tradition of presenting at least two generations of heirs.

Tiberius’ natural son Drusus Julius Casear, born in 13 BCE with the name Nero Claudius Drusus, and also known to historians as Drusus II, Drusus the Younger and Drusus Minor, was named after his paternal uncle, the general Drusus the Elder, who was Tiberius' younger brother.

Born and raised in Rome, Drusus was the first grandchild of the Augustus’ late lieutenant Marcus Vipsanius Agrippa and his first wife Caecilia Attica.

However, because Drusus is only related to the Claudian side of the family, rather than both the Julians and Claudians, Augustus had forced Tiberius to adopt Germanicus, two years Drusus’ senior, as his son and heir, removing Drusus from the succession.

Upon this adoption, Germanicus's name is changed to Germanicus Julius Caesar.

He also becomes the adoptive brother of Drusus.

Sometime between between 5 and 1 BCE, Germanicus marries his maternal second cousin Agrippina the Elder, a granddaughter of Augustus.

She is to bear him six children: Nero Caesar, Drusus Caesar, the Emperor Caligula, the Empress Agrippina the Younger, Julia Drusilla, and Julia Livilla.

Through Agrippina the Younger, Germanicus will become the maternal grandfather of the Emperor Nero.