I'm sorry you didn't know about the license requirement, but I've been planning a trip to Japan in June, and have seen this requirement almost everywhere that rental cars are mentioned (web sites, guidebooks, etc.). This morning I called Hertz to make a reservation (before I read your post), and the agent was very careful to tell me about it on the phone (maybe they improved the training in the past year since your OP).

I also am amazed that you turned around and flew straight back to the US instead of taking the trains. Was it really better to cancel your vacation than to resort to Plan B?