The Roman Empire began as a republic in 500 BCE and expanded through military conquests over centuries. It established a large territory but internal problems arose, including wealth inequality and unemployed citizens supporting generals. The republic transitioned to a dictatorship under Caesar and Octavian, ending democracy. Octavian became the emperor Augustus, though real power lay with him. The empire split but was reunited by Constantine, who also adopted Christianity. Germanic tribes then exerted pressure and invaded the Western empire, overthrowing the last emperor in 476 CE due to political, economic, and social issues.