GATE: And Thus the UNDF Fought There

43 years after an alien invasion ravages the Earth, humanity is still recovering from the invasion when a portal to another world opened in Iceland. With it comes an army seeking to conquer all before it. When the city of Reykjavik falls to this new and mysterious enemy, the United Nations Defense Foce is tasked with liberating the city, and sending a task force to the world that lay beyond.