I don't think the United states could've done much except naval interdiction and sending some marines to venezuela. I know the US had a choke hold on the region during the time, but Britain's navy outclasses the american's. So there would've been limited fighting, but bear in mind that britain and america had a good relationship at the time, they both would've really hated fighting each other over another country's borders. It probably would've been settled diplomatically at the end anyway.