The reason why northern cities saw in increase in population after the american civil war was: the flight of former slaves from the south
Following the american civil war, people in southern states still treated the ex-slaves that freed by abraham lincoln very poorly. To escape this treatment, the former slaves decided to flight to the northern states in hope for better societal treatment and job opportunities.
Even after the liberation of the slaves, the Southerners still treated them poorly, did not see them as free people, and continued with the old slave practices they had always had. Because of this, many former slaves fled from the south to the north of the country in search of better living conditions and less prejudice. For this reason, after the American Civil War, northern cities saw an increase in population
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