Which large areas were united under one ruler? Give at least three examples.

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The most powerful rulers were:

The first Persian Empire by the King of the Kings

Delhi Sultanate comprising Indian subcontinents, and the Muslim Kingdom.

British Empire which was ruled by the United Kingdom and the Empire was so big, that it was called as "the empire on which the sun never sets", due to its large occupancy on the globe.

Russian Empire which was there from 1721 to 1971. It stretched over three continents and was one of the largest empires.