Sejanus belonged to an obscure family of knights--to what we should now
call the _bourgeoisie_. He was not a senator, and he held no great
political position; for his charge as commander of the guard was a
purely military office. In ordinary times he would have remained a
secondary personage, exclusively concerned with the exacting duties of
his command; but the party of Agrippina with its intrigues, and the
weakness and uncertainty of Tiberius, made of him, however, for a
certain time, a formidable power. It is not difficult to see whence
this power arose. The loyalty of the pretorian guard, upon which
depended the security and the safety of the imperial authority, was one
of the things which must seriously have preoccupied Tiberius,
particularly in the face of the persistent and insidious intrigues and
accusations of the party of Agrippina. The guard lived at Rome, in
continual contact with the senate and the imperial house. Everything
which was said in the senatorial circles or in the palaces of the
emperor or of his relatives was quickly repeated among the cohorts, and
the memory of Drusus and Germanicus was deeply venerated by the
pretorians. If the guard could have been persuaded that the emperor
was a poisoner of his kindred, their loyalty would have been exposed to
numberless intrigues and attempts at seduction. In such a condition of
affairs, a commander of the guard who could inspire Tiberius with a
complete and absolute trust might easily acquire a great influence over
him. Sejanus knew how to inspire this trust. This was partly by
reason of his origin, for the equestrian order, on account of its
ancient rivalry with the senatorial nobility, was more favorably
inclined than the latter toward the imperial authority; and partly also
on account of certain reforms which he had succeeded in introducing
into the pretorian guard.