If it did occur, it would be a very different war, even if all the states broke the same way (not a given).
- Much smaller impact of railroads (railroad mileage tripled from 1850-1860 and was much more spread out)
- Few, if any, repeating rifles
- No ironclads
- Little use of balloons
- Less use of telegraph
So, the war would have been somewhat less centrally directed, less mobile, etc. Results would likely have been the same eventually, but it might have been harder for the Union.