The Roman Empire was an ancient state that controlled the Mediterranean and much of Europe, Western Asia, and North Africa during the classical period. The Roman Republic had previously conquered most of these territories, which later came under imperial rule following Octavian's rise to power and the establishment of the Principate in 27 BC. By the 4th century AD, the empire had divided into western and eastern halves. The Western Roman Empire collapsed in 476 AD, whereas the Eastern Roman Empire persisted until the fall of Constantinople in 1453.