The cause of the Franco-Prussian War was ostensibly a succession crisis in Spain, the Germans wanted a Hohenzollern (Prussian) prince to take the throne of Spain whereas the French were greatly opposed. Bismark used this minor succession dispute to bait Napoleon III into declaring war on Prussia, painting France as the aggressor and giving him the prefect pretext to unite the still somewhat aloof German states into an Empire. While the French and Germans warred an Italian Duke, Amedeo, was invited to take the Spanish throne, by the time the war was over German unification was complete and the Prussian claim to the Spanish throne was eventually dropped, but what if it hadn't been dropped? Amadeo didn't particularly care for the Spanish crown and two years later he abdicated after making an appearance before the Cortes (Spanish Parliament), proclaiming the Spanish people “ungovernable”. If the Germans decided to press their claims Leopold, Prince of Hohenzollern would have likely become King of Spain. Now assuming Leopold doesn't do something foolhardy to lose the crown (which in Spain means blinking the wrong way, but lets just assume he hangs in there) 1898 rolls around and the USS Maine blows up in Spanish Cuba, does the US Congress still use this as an excuse to declare war on Spain, and if they do does the German Empire declare war on the US to back their relatives in Spain?
Tl;dr: Spain gets a German king, the US starts off the Spanish–American War as per OTL, Germany declares war on the US, what happens next?
Tl;dr: Spain gets a German king, the US starts off the Spanish–American War as per OTL, Germany declares war on the US, what happens next?