Publius (or Gaius) Cornelius Tacitus (; ; – ) was a senator and a historian of the Roman Empire. The surviving portions of his two major works—the "Annals" and the "Histories"—examine the reigns of the Roman emperors Tiberius, Claudius, Nero, and those who reigned in the Year of the Four Emperors (69 AD). These two works span the history of the Roman Empire from the death of Augustus, in 14 AD, to the years of the First Jewish–Roman War, in 70 AD. There are substantial lacunae in the surviving texts, including a gap in the "Annals" that is four books long. 

Tacitus' other writings discuss oratory (in dialogue format, see "Dialogus de oratoribus"), Germania (in "De origine et situ Germanorum"), and the life of his father-in-law, Agricola, the Roman general responsible for much of the Roman conquest of Britain, mainly focusing on his campaign in Britannia ("De vita et moribus Iulii Agricolae"). 

Tacitus is considered to be one of the greatest Roman historians. He lived in what has been called the Silver Age of Latin literature, and is known for the brevity and compactness of his Latin prose, as well as for his penetrating insights into the psychology of power politics.
What aspect of it?
into the psychology of power politics