A US-German alliance after French support for the Confederacy

French support for the Confederacy could sour American relations, but a later defensive alliance with Germany would not automatically bring the United States into Germany's offensive war.

Question

What if France backed the Confederacy, the South still lost, and the United States later allied with Germany against France?

Short answer

The alliance would probably remain defensive, allowing the United States to stay out when Germany attacked, especially while American naval power remained limited.

Notes

The answer treats British naval power as a constraint on the proposed French intervention in the American Civil War. If Britain remains outside the Confederate camp, French participation is not expected to overturn the South's defeat. A later US-German alliance is assumed to be defensive, although the question does not supply its terms. On that assumption, Germany's attacks on Belgium and France could give Washington grounds to remain outside World War I, with Italy used as the comparison. American participation would also require a much larger navy and congressional funding. The discussion places much of the major naval expansion in 1916, making the timing of any earlier alternative program part of the problem.