Between the death of the emperor
Septimius Severus in 211, and the accession of Diocletian in 284, twenty-three emperors had been installed and then killed. Almost all of these emperors owed their elevation to the force of arms, and were able to maintain power so long as they both kept favour with their armed supporters, and prevented the emergence of a rival. Thus, the army and the
Praetorian Guard became the true power behind the throne. This phenomenon, where the military could act as a political force, illustrated a major constitutional dysfunction, which derived from the fact that there was no universally accepted mechanism to facilitate the installation of new emperors. The result of this chaos was the accession of the emperor
Diocletian to the throne. While the twenty-three emperors before Diocletian all encountered similar problems, the reign of the emperor
Gallienus can be used as a typical example to illustrate the state of the empire during this period. Gallienus reigned from 260 to 268, and during his reign no part of the empire (other than possibly Africa) escaped devastation at the hands of the barbarians. who often came in the form of provincial governors. The citizens in the provinces often had a common enemy, and usually could not rely on Rome for protection. Thus, their loyalty to the emperor was seriously impaired, and they turned to their governors for leadership. The governors began assuming titles that did not legally belong to them, which resulted in a virtual dismemberment of the empire. One example of these usurpers was Postumus, who was governor of
Gaul (modern France) during Gallienus' reign. Odaenathus made himself king and his wife, Zenobia, queen. Rome did not object to this, and he was given control over Asia, and was allowed to appoint governors and generals. While Odaenathus recognized the authority of the emperor Gallienus,
Zenobia did not, and in 269 she took an army and conquered Egypt. In 271, the Roman Emperor
Probus drove her out of Egypt, and quickly reacquired the province of Palmyra. ==The constitution of Diocletian==