The US isn't likely to go to war over the Panay incident, sure.
But, it COULD wake up the US establishment and military, and start a build up sooner than OTL, more soldiers in the Philippines, the sub base on Midway, better defenses on Wake, etc.
If Japan attacks the US as well as Britain, France and the Netherlands at or near the same time Hitler goes to war against them, the US might well DoW Germany at the same time as Japan - especially given the alliance between Japan and Germany.
Of course, Japan's not going to attack south in 1939. They aren't ready, they don't have the bases, etc., so that's pretty unlikely.
But, it COULD wake up the US establishment and military, and start a build up sooner than OTL, more soldiers in the Philippines, the sub base on Midway, better defenses on Wake, etc.
If Japan attacks the US as well as Britain, France and the Netherlands at or near the same time Hitler goes to war against them, the US might well DoW Germany at the same time as Japan - especially given the alliance between Japan and Germany.
Of course, Japan's not going to attack south in 1939. They aren't ready, they don't have the bases, etc., so that's pretty unlikely.