What was the principle reason why the roman empire split into two separate entities?

Emperor Diocletian split the Roman empire into two separate entities (Roman Empire and Byzantine Empire) because he believed that it would be easier to rule, defend, and allow the Empires to last longer. However, this led to constant civil war, and the weakening of the Western half, which soon fell into pieces.

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Because broadest terms allows the law flexibility depending on changing society and circumstances.