Pertinax, an aged senator of consular rank, and now Præfect of the city, was summoned by the conspirators, who came to his house late at night, after the murder of Commodus, to ascend the vacant throne. He was one of the few friends and ministers of Marcus Aurelius who yet survived, and, having filled many important offices, had always been distinguished for firmness, prudence, and integrity. The rumor was spread that Commodus had died of apoplexy, and that Pertinax had succeeded him; but the Prætorian Guards were dissatisfied at his election. The Senate, however, confirmed the choice of the conspirators, and Pertinax lived among his own order rather as an equal than a master. His manners were simple, his mode of life frugal, and he sought to revive the pleasing simplicity of the early Republic.

Pertinax administered justice with strictness, released those who had been left in prison by Commodus, reformed the finances and introduced economy, redivided the uncultivated lands among those who would till them, removed oppressive restrictions upon trade, and deserved the respect of the wiser portion of his subjects.

But the Prætorians were never reconciled to his rule, and on the 28th of March, A.D. 193, eighty-six days after his election, they broke into the imperial palace, and struck down the emperor with innumerable blows. His head was separated from his body, and, being placed upon a lance, was carried in triumph to the Prætorian camp, while the people silently lamented the death of this virtuous ruler.

The soldiers, meanwhile, proclaimed from the ramparts of their camp that the throne of the world would be sold at auction to the highest bidder. Didius Julianus, a wealthy Senator, whose age had not quenched his vanity and ambition, offered about a thousand dollars to each man for the possession of the prize. He was declared emperor, and, surrounded by the armed Prætorians, was carried to the Senate, who were forced to accept the selection of the soldiers. But the Senators and the people felt deeply the disgrace of their country, and even the Prætorians were ashamed of their unworthy choice. Julianus found himself on the throne of the world without a friend.

The armies in the provinces, when they heard of these transactions at the capital, rose in revolt, and refused to acknowledge the authority of Julian. Clodius Albinus commanded the legions in Britain, Septimius Severus those in Pannonia, and Pescennius Niger the army of the East. Severus, more active than his competitors, was saluted by his soldiers as emperor, and marched rapidly toward Rome. Julian, deserted by the Prætorians, was condemned to death by the Senate, and was executed as a common criminal after a reign of only sixty-six days. Severus was acknowledged as their lawful emperor by the Senate, June 2, A.D. 193, and his first act was to disarm the Prætorian Guards and banish them from the capital.

He next marched against Niger, and defeated him in two battles, while he was also successful in a severe contest with Clodius Albinus at Lyons. Both of his competitors were put to death, and Severus, now set free from fear of rivalry, began to show the native cruelty of his disposition. Forty-one Senators, whom he accused of having favored Albinus, were executed, with their wives and children; and many of the provincial nobles of Spain and Gaul shared their fate. Yet Severus was in many respects a useful ruler; strict in the administration of the laws, careful to correct abuses, and restraining his subjects with stern impartiality. Peace returned to the provinces, cities were repeopled, roads repaired, Rome abounded in provisions, and the people were satisfied. Severus changed the constitution of the Prætorian Guards, and filled up their ranks with the bravest soldiers of the legions of the frontier. These barbarians, he thought, would be able to suppress any rebellion that might arise; and he increased the number to fifty thousand men. The Præfect of the Prætorians, who had at first been a simple soldier, now became the chief minister of the emperor, and was at the head of the finances and even of the law. The celebrated lawyer Papinian was appointed Præfect after the fall of Plautianus; and several great jurisconsults, particularly Paulus and Ulpian, flourished under the reign of Severus or his family.

Severus, however, was a military despot, and, despising the feeble Senate, assumed both the legislative and the executive power. The jurisconsults, in fact, from this reign, begin to treat the emperor as the source of all law, the Senate and the people being no longer considered in the state. But this arbitrary rule, introduced by Severus, is thought to have tended more than any thing else to destroy the vigor of the Roman Empire, by leading the people to an abject dependence upon their rulers.

The wife of Severus, Julia Domna, a Syrian lady of great beauty and various accomplishments, became the mother of two sons, Caracalla and Geta, and the emperor hoped that they would prove worthy of the high office to which they were born. They soon, however, showed themselves incapable of any serious study or employment, and were chiefly remarkable for the hatred they bore toward each other. The court was already divided into two factions, composed of the adherents of either son; and the emperor, who in vain strove to remove their rivalry, foresaw that one must fall a victim to the hatred of the other.

In A.D. 208 a war broke out in Britain, and Severus, although now more than sixty years of age, and afflicted with the gout, so that he was carried on a litter, set out at the head of his army, attended by his two sons, and penetrated into the interior of Scotland. This was his last enterprise, for he died at York, February 4, A.D. 211. He left his empire to his two sons, who returned to Rome, and were acknowledged by the Senate and the army.