The Annals and the Histories · Ann. 3.56

Tiberius having gained credit for forbearance by the check he had given to the growing terror of the informers, wrote a letter to the Senate requesting the tribunitian power for Drusus. This was a phrase which Augustus devised as a designation of supremacy, so that without assuming the name of king or dictator he might have some title to mark his elevation above all other authority. He then chose Marcus Agrippa to be his associate in this power, and on Agrippa's death, Tiberius Nero, that there might be no uncertainty as to the succession. In this manner he thought to check the perverse ambition of others, while he had confidence in Nero's moderation and in his own greatness. Following this precedent, Tiberius now placed Drusus next to the throne, though while Germanicus was alive he had maintained an impartial attitude towards the two princes. However in the beginning of his letter he…

Cornelius Tacitus · trans. Church & Brodribb, 1876
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