The Crisis of the Third Century in the Roman Empire lasted fifty years, from the death of Severus Alexander to the accession of Diocletian. Rome was near collapse in all spheres: social, political, military, and economic. There was anarchy, rebellions, and the violent deaths of rulers.
The causes of the crisis provide some insight into the outcome. One of the main causes of the crisis was was the absence of a clear means of succession, sometimes referred to as the "constitutional problem." Would succession be patrilineal? Would the reigning emperor choose? Would the Senate elect the ruler? Would he be selected by the Praetorian Guard? A second cause was the absence of an efficient provincial administration. Rome was a "federation of cities,” with the curia run by the aristocracy. The imperial government stepped in to fill the void, which sapped its resources. The state responded by increasing taxes, which led to popular discontent.
Severus Alexander sought a negotiated truce with Alamanni but was killed by his troops and replaced by Maximinus Thrax. Maximinus, of peasant origin, was the first of the ”barracks emperors.” Civil war ensued, with short unstable reigns: Maximinus, then Gordian III, then Philip, then Decius (249-51), and so forth.
Some historians point to the destabilizing effect of the growth of Christianity in the realm. That perspective was adopted by some of Rome's rulers, who saw the crisis as the wrath of the gods, and this led to the persecution of Christians. The Decian persecution was the first formal campaign, although the seventh in total, against Christians since Nero in the 60s A.D. Decius' failure to restore order, coupled with the example of martyrdom, held up as an ideal by Christians, likely led to the growth of the religion in the empire.
The crisis ended with the election of another "barracks emperor," Diocletian, in 284. He was chosen by the army of the East and, then, after the death of Carius in battle in 285, by the army of the West.
Diocletian's reforms, since they sought to address problems that predated his ascent, might all be seen as an effect of the Crisis of the Third Century. Realizing the enormity of the empire and the challenges it was facing, he chose Maximian to rule jointly with him. At first, Diocletian named Maximian Caesar and co-emperor; later he rewarded him for his military successes with the rank of augustus. Maximian defeated a rebellion in Gaul and repelled German incursions into Gaul. Meanwhile, Diocletian stayed focused on the East. Given continuing military challenges, and namely the problem with the rebel Carausius in Britain and northwest Gaul, Diocletian extended collegial rule. Each Augustus, he and Maximian, would have a Caesar or junior emperor. This “rule of four” is referred to as the Tetrarchy. Galerius became Diocletian's co-emperor in the East and Constantius ruled with Maximian in the West. In this way, he addressed the issue of succession: junior emperors would eventually become senior emperors, as the senior emperors would voluntarily retire. The promise of promotion gave the junior emperors a stake in the success of the state.
Diocletian also reformed the administrative apparatus and grew the bureaucracy. He separated the civil administration from the military. He increased the number of administrative subdivisions in the empire. The provinces were subdivided and made smaller, doubling from fifty to one hundred. Provinces became part of a larger unit called a diocese, of which there were twelve. There were four praetorian prefects who answered to the emperor.
He also launched some important military and economic reforms to deal with armed threats and the empty treasury, respectively. He established mobile field armies that were always ready to move. He based the tax in kind on potential agricultural production, rather than actual production, to encourage productivity and held the local curia responsible for tax payments. He fixed maximum prices for goods and issued the gold aureus to try to stem inflation. Finally, he oversaw the Great Persecution (302-05), deeming Christians to be a problem, as had Decius. Diocletian's policies all sought to address many of the problems that arose from and during the Crisis of the Third Century.