I think the Japanese are being a bit dumb here, and could have helped possibly avoid this situation.
Considering that they've been in this world long enough to build railroads and oil rigs, and the air travel capabilities they have, they should have sent diplomatic first-contact missions to multiple countries by now to get more trading partners, show off their toys a bit, and casually mention "by the way, these folks are our allies" to the nations that are periodically at war with the nations supplying essential resources to their islands. They don't have to mention it's just a trading alliance, or that they're not legally allowed to send troops in.
That would at least possibly cause these nutjobs to think twice about pulling an invasion, because it changes the strategic situation vastly from "oh, the new island's just filled with unadvanced barbarians", as their king said last chapter. (His spies have to be truly incompetent to not notice the railway Japan's been building.)
But no, that meant we wouldn't get an irrational hate-fueled enemy to demonize and give us an excuse to do the dragons vs. jets thing...