The biggest impact of the civil war was the end of slavery.
Step-by-step explanation:
The southern states had been a little aloof from the US for a long time.
This was partially because f the 'peculiar practice' and many other such things.
These were solved by might and the southern states, albeit with some resentment were brought back into the fold of the union.
At the same time, the south had to give away slavery and then the freed men also became citizens and started having an influence on the policies, albeit a minuscule one.