Film
I read (not sure of the source, so I can't speak to its accuracy) that there was film that wasn't released at the time. If newsreels through the country showed American sailors gunned down by Japanese fighters- includnig an especially gory one of someone taking a direct hit from a large calliber round, bloody, mangled corpses, the gunboat going down, colors flying as the last gun spits defiance at the Japanese...perhaps with an ememy plane going down, there would be a more energetic reaction throughout the USA. Then, the politics, media, and general mood of the popluation (that influenced by the politics and media) could spiral out of control.
If the USA is already mobilized for war in 1939, I suspect that World War II starts on schedule, but that the Germans are a bit more careful in dealing with the USA. By 1940, it's clear that the USA can build like crazy, and Germany would not want to see American shipbuilding pointed at them once Japan is contained. Even more, if peace breaks out in the Pacific around 1940, the USA has lots of suddenly surplus light ships for Britian...