I have been accused of being a lot of things on this BB..."not reading", "a bitter old woman", "a turn coat", of being “closed minded”...WAIT, the last one I've said that of myself! (Scratching head-puzzled look emoticon goes here!)
"why was the United States the only country that needed a war to end slavery."
Something had to give, and it wasn't going to be the South. The South put up with the Tariff nonsense for a number of years, but it was going to cut into their Economic and Social structure to give up slavery. That was going to affect every one, no matter how high or low their standing in society was.
I firmly believe that is the big reason the South Seceded, to get the slavery issue back to the States, away from a centralizing Government. That way it would for a long time be guaranteed. It would be much harder to do away with slavery if individual states had control, than a all encompassing central Government saying it was abolished.
The South secedes to keep slavery, the North attacks the South (when fired upon by the South) because the North is against secession…it all embroils down to one big slaughter, the reasons covered with the blood of both both sides.