When did start the Byzantine Empire?

In the 3rd and 4th centuries AD, the Roman Empire began to face increasingly pressing challenges. External threats from barbarian invasions, especially along the northern and eastern frontiers, placed immense pressure on military resources, making it difficult to maintain control over distant provinces and govern effectively from a single center. These challenges were exacerbated by political instability, power struggles, and succession crises.

Additionally, the Western and Eastern regions of the empire were markedly different, with divergent interests. While the Eastern regions were prosperous, urbanized, and influenced by Greek culture, the Western regions were primarily agrarian-based, experiencing economic and demographic decline, and romanized.

This division in interests and culture led to a political division within the empire, a trend that had already been seen in the 3rd century with the establishment of the Tetrarchy, a political system dividing the empire into four parts. By the late 4th century, the empire was formally split into two entities: the Eastern Roman Empire, with its capital at Constantinople, and the Western Roman Empire. In 476 AD, Odoacer deposed the last Western Roman emperor, Romulus Augustulus, and returned the regalia of the Roman Empire to Constantinople. With this act, the Eastern Roman Empire emerged as the sole legitimate successor to the Roman Empire and would henceforth be known as the Byzantine Empire. It would endure for a thousand years, through a rich, turbulent, and complex history.