After the death of the replacement bishop Gregory in 345, Constans used his influence to allow Athanasius to return to Alexandria in October 345, amidst the enthusiastic demonstrations of the populace. This began a "golden decade" of peace and prosperity, during which time Athanasius assembled several documents relating to his exiles and returns from exile in the Apology Against the Arians. However, upon Constans's death in 350, another civil war broke out, which left pro-Arian Constantius as sole emperor. An Alexandria local council in 350 replaced (or reaffirmed) Athanasius in his see.
What happened when Constans died?
civil war