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A. The threat of another Persian Invasion

Explanation:

They united under one government because they were afraid of another Persian Invasion, which ended up happening anyway in 480 BC.  Xerxes personally led the second Persian invasion of Greece with one of the largest ancient armies ever assembled. Victory over the allied Greek states at the famous Battle of Thermopylae allowed the Persians to torch an evacuated Athens and overrun most of Greece.

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answer; the period of time known as the middle ages;