Which Harry Turtledove spoofs in one of his many[1] novel series taking a twist on the World Wars / the second World War, namely the one where they're mainly fo

Which Harry Turtledove spoofs in one of his many[1] novel series taking a twist on the World Wars / the second World War, namely the one where they're mainly fought in America between the USA and a CSA which in that timeline remained independent after winning the Civil War. There', they're called "barrels". ___ [1]: At least three including this, but I have a feeling I may be forgetting at least one more.