Gate - Jietai Kare no Chi nite, Kaku Tatakeri

One day in the 21st century, a gate opened in the middle of the Ginza district of Tokyo. Out of the gate poured soldiers and strange creatures, and they soon made Ginza look like an image of hell. The gate apparently connected Japan to another world, a world with a militant empire and many supernatural creatures. The Japanese Self-Defense Force decided to send a small group, led by a soldier who's also an otaku, into the other world on a reconnaissance mission. They're meant to learn what they can about the empire (and its plans), the world, and the people.