Soon after taking power, Diocletian named a colleague, Maximian, Caesar and assigned him control of the western provinces. This act gave him free reign to deal with problems along the Danube – five years worth. The dual-emperor model worked well but did nothing to solve the problem of succession. Diocletian fixed that problem in 293 A.D.by having Maximian and himself name their replacements – Julius Constantius and Galerius Maximianus.
All four of them had been participating in the wars starting in 286 A.D: a Berber revolt in Africa, a Persian seizure of Armenia, a pretender in Egypt declaring himself emperor, and a breakaway commander in Britain. These challenges took four years to clean up. The Goths and Germans were also troublesome during this time, but they were held back by a superior Roman effort and dedicated commanders. The Germans spent much of the time fighting among themselves.