What was a major result of the Civil War on the US?
The Union was preserved as a single country, and slavery was abolished. Is that major enough?
In addition to the answers drwls gave, you might consider the total change of America's status in the world. REconstruction was one result that was not a good one. Ex-slaves had their freedom, but found that economics following the war were poor in the south, and many whites also entered the ranks of the poor, never to escape. Many Southerners fled west to territories that were sympathetic to their philosophies. Texas drew many ex-soldiers. Without the Civil War, settling the west might have taken longer.