Send a couple of Type B1 submarines to the West Coast, preferably to LA or San Francisco, then have them hit civilian targets. The real goal isn't to cause damage; given that each submarine carries a single seaplane, that would be futile. The goal is to force the US to assign additional resources to protecting the West Coast that might otherwise have been sent to the actual battlefields in the Pacific. Try to send at least one or two subs out for that purpose every month, just to keep a nice simmering paranoia and needless fear of invasion going in California, Oregon and Washington.