Original sentence: Famous examples are Alexander the Great and his empire, the Mongol Empire, and the British Empire, which all encompassed substantial portions of the then known world; in the case of the British, a quarter of the world's surface and approximately a third of the world's population
Edited sentence: Famous examples are Alexander the Great and his empire, the Mongol Empire, and the British Empire (However it should be noted that the United Kingdom itself was a Democracy), which all encompassed substantial portions of the then known world; in the case of the British, a quarter of the world's surface and approximately a third of the world's population