Sounds like Harry Turtledove's "Must and Shall" ...Well, you could possibly have rebellion / sesession in the south during ww1, assuming a much messier end to the CW and reconstruction turning into repression and a 50 year low level civil war - think Algeria writ large.
In such a scenario US politics is VERY different at least from '76 onwards...
It would also be a much messier and unpleasent place.
However the only way you can get significant WW1 european intervention is if the US is already in the war, THEN rebellion in the south and THEN which ever power(s) the US is at war with sending help to the CSA.
Of course that only works if the US is on the CP side, as the entente controls the sealanes...
Lincoln is Shot, During The Battle of Fort Stevens, And Hannibal Hamlin Exacts a Draconian Peace ...
One of The Throwaway Lines, is The Central Powers Aiding The Still-Simmering Rebellion, in The American South!