The next seventeen years (251-268) is a period of great confusion. Several generals in different provinces were declared Emperor. The Empire nearly fell to pieces, but finally rallied without loss of territory. Its weakness, however, was apparent to all. This period is often called the AGE OF THE THIRTY TYRANTS. FIVE GOOD EMPERORS (268-283). FIVE GOOD EMPERORS now ruled and revived somewhat the shattered strength of the government: CLAUDIUS (268-270); AURELIAN (270-275); TACITUS (275-276); PROBUS (276-282); and CARUS (282-283). Aurelian undertook a campaign against the famous ZENOBIA, Queen of PALMYRA. In her he found a worthy foe, one whose political ability was rendered more brilliant by her justice and courage. Defeated in the field, she fortified herself in Palmyra, which was taken after a siege and destroyed. Zenobia was carried to Rome, where she graced the triumph of her conqueror, but was afterwards permitted to live in retirement. Aurelian was the first who built the walls of Rome in their present position. DIOCLETIAN (284-305). With this ruler, the last vestige of the old republican form of government at Rome disappears. Old Rome was dead. Her Senate had lost the last remnant of its respectability. Seeing the necessity of a more united country and a firmer rule, DIOCLETIAN associated with himself MAXIMIAN, a gigantic soldier, who signalized his accession by subduing a dangerous revolt in Gaul. He also appointed two officers, GALERIUS and CONSTANTIUS, whom he called CAESARS,--one to have charge of the East, and the other of the West. By means of these assistants he crushed all revolts, strengthened the waning power of the Empire, and imposed peace and good order upon the world. Diocletian and Maximian afterwards resigned, and allowed their two Caesars to assume the rank of AUGUSTI, and they in their turn appointed Caesars as assistants. Soon after his accession Constantius died, and his son CONSTANTINE was proclaimed Caesar, against the wishes of Galerius. A bitter struggle followed, in which Constantine finally overcame all his opponents, and was declared sole Emperor. For his successes he was named the GREAT. CONSTANTINE THE GREAT (306-337).