1. Geographic reasons
Empires that span a large geographic area can be difficult to govern from a central location. Dividing the empire into two or more parts can make it easier to administer. For example, the Roman Empire was divided into the Western Roman Empire and the Eastern Roman Empire in the 4th century AD. The Western Roman Empire collapsed in the 5th century AD, but the Eastern Roman Empire continued to exist until the 15th century AD.
2. Cultural reasons
Empires that are composed of different cultures can be difficult to unite under a single government. Dividing the empire into two or more parts can allow each part to maintain its own culture and identity. For example, the British Empire was divided into the United Kingdom, Canada, Australia, New Zealand, and South Africa. Each of these countries has its own unique culture and identity, but they are all united under the British monarchy.
3. Political reasons
Empires can be divided for political reasons, such as to strengthen the power of the central government or to weaken the power of a rival empire. For example, the Ottoman Empire was divided into the Republic of Turkey and the Republic of Iraq after World War I. The division of the Ottoman Empire was intended to weaken the power of the Ottoman Empire and to create new nation-states in the Middle East.
4. Economic reasons
Empires can be divided for economic reasons, such as to improve trade or to reduce the cost of governing. For example, the Spanish Empire was divided into the Viceroyalty of New Spain and the Viceroyalty of Peru in the 16th century. The division of the Spanish Empire was intended to improve trade between Spain and its colonies in the Americas.