Which emperor permanently divided the Roman Empire into Eastern and Western halves? (r. 379-395 AD)

The story behind the answer

Theodosius I is correct because, upon his death in 395 AD, he left the Roman Empire to his two sons—Arcadius in the East and Honorius in the West—creating a permanent political split. He was the last emperor to rule both halves as a single entity.

Context: Earlier, Diocletian had introduced a temporary administrative division (the Tetrarchy), but Theodosius’s succession plan made the separation enduring. He also cemented Nicene Christianity as the state religion (Edict of Thessalonica