Considered the turning point of the war because it was the last time the south invaded the north

The Battle of Gettysburg.

The Battle of Gettysburg was the south's last attempt in pushing the war into the northern states, as wars in the south displaced people, allowed the Northern armies to destroy much needed crops, economy, and what little means of production the South had. However, at the end, the South still lost, and it was a road to defeat soon after.

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nothing certain is known about what happened, but it seems that the country declined gradually, perhaps due to bad government, or, more likely, due to a series of bad harvests, which resulted in widespread and recurrent famine.

a social contract.

"a social contract is an agreement between citizens and their leader where the citizens give up certain freedoms in exchange for protection. the constitution of the united states is an example of a social contract."