Tacitus was succeeded by his half-brother Florian, who was almost immediately challenged by the general Probus. Although Probus was only recognized in a few eastern provinces, and had a much smaller army, he quickly outmaneuvered and deposed Florian to become the sole emperor in September 276.
The Agri Decumates was lost to the Romans in c.262, regained by Aurelian and Probus in 275–8, and lost again sometime between 290 and 310. The losses here seem not so much due to any rise in power of the local Alemanni tribe, but Roman internal division. When rival Roman factions controlled Gaul and Raetia, as was the case in 262–274 and 306, the limes of the Agri Decumates were no longer defensible and had to be abandoned.
After the death of Tacitus, the army in Asia Minor chose his half-brother Florian to succeed him as emperor in June 276. Florian seems to have been quickly recognized by the Roman Senate, but was unable to achieve much—and most likely never returned to Rome—before Aurelian’s general Probus revolted in Syria. After just over two months’ rule, Florian was either killed by his own troops or forced to commit suicide by opening his veins.in wikipedia
When the eastern legions learned of the death of Tacitus and his succession by Florian in June 276, they proclaimed Marcus Aurelius Probus—their general and a veteran of Aurelian’s restoration wars—as emperor. Probus quickly gained recognition in Syria, Phoenice, Palestina, and Egypt, but lacked both the troop numbers and wider support of his rival Florian.in wikipedia
When Florian heard of Probus’ claim, he abandoned his pursuit of the Heruli—allowing these invaders to escape across the Black Sea—and headed south to Tarsus with his army. Outnumbered by his rival, Probus avoided direct combat until Florian’s troops—who were mostly European and therefore unaccustomed to the climate of Cilicia—began suffering from the summer heat and disease. Probus then attacked and, easily defeating his weakened enemy outside Tarsus, deposed Florian.in wikipedia