A.H.C.: U.S.A. and C.S.A. are allied in a World War

Assuming that...
  1. The Confederate secession is successful and the C.S.A. survives into the 1900s
  2. The European "powder keg" explodes sometime in the early 20th Century
...would it be possible not only to involve both the Union and the Confederacy, but have them on the same side, either as full-fledged allies or at least co-belligerents?

I know the common AH trope is that one will join the Central Powers and one will join the Entente.
 
Actually, I think it's perfectly plausible that both will join the Allies, since Germany is likely to apply unlimited submarine warfare to both. (If she applies it only to the US, it will be too easy for American war goods to reach the Allies via the Confederacy.)

And not all AH works have one republic joining the Allies and the other the Central Powers: "In If the South Had Won the Civil War by MacKinlay Kantor, reunification comes later: during the 20th Century the USA, CSA and Texas (which seceded from the CSA) become economically integrated and in both World Wars, they all fight against Germany as close allies. Following World War II, USA, CSA and Texas all feel threatened by Soviet missile bases and armored brigades in Alaska (which was never purchased from Russia). Therefore, they announce a formal reunification in 1961, on the precise centennial of Fort Sumter." https://en.wikipedia.org/wiki/American_Civil_War_alternate_histories
 
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