Constantius II was Roman Emperor from 337 to 361. The second son of Constantine I and Fausta, he ascended to the throne with his brothers Constantine II and Constans upon their father's death.
In 340, Constantius' brothers clashed over the western provinces of the empire. The resulting conflict left Constantine II dead and Constans as ruler of the west until he was overthrown and assassinated in 350 by the usurper Magnentius. Unwilling to accept Magnentius as co-ruler, Constantius marched against him. Magnentius was defeated at the battles of Mursa Major and Mons Seleucus, committing suicide after the latter. This left Constantius as sole ruler of the empire.
His subsequent military campaigns against Germanic tribes were successful: he defeated the Alamanni in 354, and campaigned across the Danube against the Quadi and Sarmatians in 357. Contrastingly, in the east, the two decade old war against the Sassanids continued with mixed results.
As a consequence of the difficulty of managing the entire empire alone, Constantius elevated two of his cousins to the subordinate rank of Caesar. Constantius Gallus, the eldest surviving son of Constantius' half-uncle, Julius Constantius, was promoted in 351, but executed three years later for his supposedly violent and corrupt nature. Constantius then promoted Gallus' younger half-brother, Julian, who was Constantius' last surviving cousin and the man who would ultimately succeed him, in 355.
However, the actions of Julian in claiming the rank of Augustus in 360 led to war between the two. Ultimately, no battle ever took place as Constantius became ill and died late in 361, though not before naming his opponent as his successor.