Not sure what a good casus belli would be. Perhaps the Germans trying to expand their influence in the Caribbean. Neither side has the lift capacity or the logistics to undertake a major ground campaign in each other's territories, so the war is mainly a naval affair. I see the U.S. sitting in the Western Atlantic, daring the Germans to come after them. The Germans, recognizing that they have no support bases in the Atlantic, send U-Boats out to raid the U.S. shipping. Honestly, I don't see this war lasting long as it's in neither the U.S. nor Germany's interests to fight.