Your challenge is with a POD after Appomatox for the USA to lose enough land in a war/treaty that regaining the land is part of the political platform of a party that takes at least 5% of the vote in a US Presidential election. (This can be 20th century as well, but I figured with the 1865 limit, I'd put it here).
naraht
I thought at 1st that you were referring to a south winning independence but having read more carefully I take it you mean one where the US loses land in a later conflict.
Only thing I can think of is some clash with Britain which goes badly for the US. Some dispute over the borders with Canada or possibly a Venezuelan crisis that spirals into a major war. It's really got to be with Britain as it's the only power before the end of WWII with the reach to actually threaten defeating the US in N America, plus any power from Europe would almost certainly have to get past Britain 1st.
Might have something a bit later, possibly a 1920-30's clash with a continued Anglo-Japanese alliance, although you would need a shorter WWI for that I think.
After that, short of somehow the Soviets defeat the US in a nuclear exchange or a new successful succession, both of which are verging on ASB without drastic changes.
If the US suffered a serious defeat and lost land as a result, especially in it's 48 state continental heartland, I could see revanchists feeling being a powerful element in the US even if the defeat is hard enough to make most people think otherwise.
Steve