Why was the Civil War considered a turning point in American history?
It resolved the questions of states rights and slavery.
With the Civil War won by the Union slavery came to an end in practice. In law it took a few more years for the passage of the 13th Amendment.
From the beginning of the United States there had been a continual discussion of the power of the federal government and the power of each state. In the case of the southern states, in 1861 the looked upon themselves as autonomous and had the right to withdraw from the union if they so decided. The northern states contended that each state was subject to federal law and the US Constitution.
In the end the federal government prevailed in the claim that it alone was sovereign and the individual states are not, as all southern states had claimed.
Because of the Civil war the 13th, 14th, and 15th Amendments were passed. The 13th ended slavery, 14th guaranteed equal protection under the law, and 15th made illegal the denial of voting rights to all men regardless of race.