[quote=@TomeBinder]
If not, I'm happy to hear your ideas.[/quote]

Or maybe his background could change a little. Maybe he's been dispatched to the US as part of an exchange. Japan needs something for its fight and the US needed something for it. I'm thinking since there's plenty of US troops in Japan, maybe Japan decided to take them for its own purposes, and in exchange provide successfully tried and tested specialist ground troops (Kyonshikirapeitai, or zombie killer unit) and support in the UN for a grand scheme involving liberating the US of zombies before she sends troop all over the world to save the world? It's not a bad deal, considering that the US military in Japanese waters and soil would have powerful air force and navy, which would be an awesome asset. Whether it helped or even lasted, however, I'm sure is up to you.

Then of course, things didn't go as planned in the US when they made landfall. While they were unequaled in sending the dead back to their grave, there's simply too many of them. Eventually, like everyone else, they were overwhelmed and forced to scatter and mount resistance in small enclaves.