Constantius I vs Deiotarus of Galatia

Constantius I led an invasion to reclaim Britain from the usurper Allectus, who had succeeded Carausius. His forces landed near London and defeated Allectus, restoring Britain to the Roman Empire and ending the breakaway Britannic Empire.

Constantius I defeated the Alemanni at the Battle of Lingones (modern Langres) and later at Vindonissa. These victories secured the Rhine frontier and earned him the title Germanicus Maximus, strengthening his reputation as a capable military commander.

Upon the abdication of Diocletian and Maximian, Constantius I became the senior Augustus of the Western Roman Empire as part of the Tetrarchy. He ruled jointly with Galerius in the East, but his reign was short-lived as he died the following year.

Constantius I died at Eboracum (modern York) while on campaign against the Picts in northern Britain. His son Constantine was proclaimed Augustus by the troops, setting the stage for Constantine's eventual rise to sole emperor.

Constantius I, unlike his co-emperor Galerius, was tolerant of Christians and did not enforce the Diocletianic persecution in his territories. This policy of religious tolerance foreshadowed the Edict of Milan issued by his son Constantine.

Deiotarus of Galatia was accused of plotting to assassinate Julius Caesar. The orator Cicero defended him in a speech before the Roman Senate, arguing his loyalty. Deiotarus was acquitted, retaining his kingdom.