Unrestricted German sub warfare in the Atlantic made it our business
Remember the Lusitania
Because the Brits had a naval blockade of Germany, so our goods couldn’t get through to Germany.
While the British government denied it for decades, we now know that the Lusitania was, in fact, carrying munitions (including 4 million rounds of .303). The German government also took out ads in American newspapers to warn potential passengers about it. These ads ran right next to Cunard’s ads for the Lusitania. We also know that through his private letters that
Churchill hoped to embroil neutral powers into the war (and, in particular the United States) through allowing catastrophes like the one that happened to the Lusitania. It’s not exactly a sound casus belli in retrospect, I’d go so far as to say that it’s an important reminder that the “deep state” was with us, even back then.