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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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was a long, primarily nonviolent series of events to bring full civil rights and equality under the law to all americans. the era has had a lasting impact on united states society, in its tactics, the increased social and legal acceptance of civil rights, and in its exposure of the prevalence and cost of racism. in the south violence and fraud in southern elections from 1868 to 1876, which had reduced black voter turnout and enabled southern white democrats to regain power in state legislatures across the south