The Romans did not establish precise rules for the succession of
emperors. Consequently, emperors came to be chosen in several ways:
designation by the predecessor from within imperial families,
designation of an adopted son by the incumbent emperor, designation
of a heir apparent from within an imperial family, designation by
the senate, proclamation by the military, civil wars, and
usurpations. Many emperors were assassinated.
The five emperors of the first dynasty, the Julio-Claudians
(27-BC-68 AD), were chosen by their predecessors from members of
these two families, apart from Claudius who was chosen by the
imperial guard after they murdered Caligula. In 69 Vespasian became
emperor by winning a civil war and established the three-emperor
dynasty of the Flavians (69-96). The other two emperors were his
two sons (Titus and Domitian) who succeeded each other. Nerva, a
diplomat, was chosen in 96 by the senate after Domitian was
murdered by courtiers. Four of the other five emperors of this
dynasty (the Nerva- Antonine, 96-192) were adopted by their
predecessors. The last one was a natural son. Septimius Severus
became emperor by winning two civil wars and established the
Severan dynasty (193-235). He saw succeeded by his two sons and one
of then, Caracalla, murdered his brother. Caracalla was murdered
and the senate chose Macrinus, a court bureaucrat, as a caretaker
emperor. Elagabalus, an in-law relative of Septimius Severus made a
bid for power ad defeated Macrinus in battle. He was proclaimed
emperor by a previously hostile senate. Elagabalus was also
assassinated and was succeeded by his cousin.
In the third century there was a period of military anarchy. The
army came under great strain by a series of invasions a in a great
part of the vast frontiers of the empire. The invasions were
repelled, but at a high cost. There was military unrest. Most of
the emperors in this period were military commanders who were
proclaimed emperors by their own troops. The turnover of emperors
was fast. Many emperors were murdered by troops, sometimes even by
the troops which had proclaimed them emperor. There were also
usurper emperors.
In 284 Diocletian was proclaimed by the troops and restored
peace. After this Constantine the Great became emperor by winning
two civil wars. The Constantinian dynasty lasted from 324 to 363.
When its last emperor died, Valentinian I, a military commander,
was proclaimed emperor by the army. This led to the Valentinian
dynasty (364-378). The Theodosian dynasty (378-457) was established
by Theodosius the Great who had been made co-emperor by one of the
Valentinian emperors.
