Gate Zero: Jieitai Ginza nite

In the summer of 20XX, a "gate to another world'' suddenly appeared in Ginza, Tokyo. What came pouring in from inside was a mysterious army and a group of hideous monsters. They go around slaughtering people, and Ginza in broad daylight quickly turns into a hellscape. While everyone was helplessly thrown into confusion in this state of emergency, there was one man who was brave enough to move around. That man was Yohji Itami, an otaku Self-Defense Force officer who would later be called the "Hero of Nijubashi"!

In the year 20XX, a mysterious gate to another world suddenly opens in Tokyo’s Ginza district. From beyond it pour invading armies and monstrous creatures from a fantasy realm. Japan’s Self-Defense Forces repel the attack-and push forward through the gate into the unknown territory on the other side. Among the soldiers sent to investigate is Yoji Itami, a 33-year-old otaku officer who is anything but a typical soldier. Leading his unit into the newly discovered world, he sets out to scout nearby villages and assess the situation beyond the gate. What awaits them is not only danger¿ but the sight of fantastical races and beautiful girls straight out of a dream. Thus begins the unprecedented encounter between modern military force and a medieval fantasy world, where guns meet dragons and diplomacy clashes with magic. *Source: Hanashi Media, Vol. 1*