With the effect of Lincoln's plans to reunify the country, the start of the Reconstruction era, when former Southern rebel states were reintegrated into the Union, is the cause.
More about Reconstruction era:
The Reconstruction era, which lasted from 1865 to 1877 after the American Civil War, saw the country struggle to decide how to treat African Americans legally and to reintegrate the seceded states into the Union.As ordered by Congress, Reconstruction removed the last vestiges of Confederate secession in the Southern states while also abolishing slavery. Reconstruction also refers to Congress's overall effort to change the 11 former Confederate states and the Union states' contribution to that change.
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The believe that the answer to the question asked above is that leaders responded by looking for more resources to be superior to others, even come to a point invading others for their wealth.
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Egyptian idk I just guessed
World war three is the answer