Constantius II (Latin: Flavius Julius Constantius Augustus; August 7, 317 – November 3, 361), is Roman Emperor from 337 to 361.

The second son of Constantine I and Fausta, he ascends to the throne with his brothers Constantine II and Constans upon their father's death.

In 340, Constantius' brothers clash over the western provinces of the empire.

The resulting conflict leaves Constantine II dead and Constans as ruler of the west until he is overthrown and assassinated in 350 by the usurper Magnentius.

Unwilling to accept Magnentius as co-ruler, Constantius marches against him.

Magnentius is defeated at the battles of Mursa Major and Mons Seleucus, committing suicide after the latter.

This leaves Constantius as sole ruler of the empire.

His subsequent military campaigns against Germanic tribes are successful: he defeats the Alamanni in 354, and campaigns across the Danube against the Quadi and Sarmatians in 357.

Contrastingly, in the east, the two decade old war against the Sassanids continues with mixed results.

As a consequence of the difficulty of managing the entire empire alone, Constantius elevates two of his cousins to the subordinate rank of Caesar.

Constantius Gallus, the eldest surviving son of Constantius' half-uncle, Julius Constantius, is promoted in 351, but executed three years later for his supposedly violent and corrupt nature.

Constantius then promotes Gallus' younger half-brother, Julian, who is Constantius' last surviving cousin and the man who will ultimately succeed him, in 355.

However, the actions of Julian in claiming the rank of Augustus in 360 lead to war between the two.

Ultimately, no battle ever takes place as Constantius becomes ill and dies late in 361, though not before naming his opponent as his successor.