GATE - Thus the JSDF Fought There!

When a portal from another world appears in Ginza, Tokyo, a
legion of soldiers and monsters emerge to attack the city. Thanks
to their far more advanced weaponry and tactics, the Japan SelfDefense Forces (JSDF) easily repels the enemy, passes through the
gate and establish a forward base of operations to force the
Roman-styled empire of the other world to open peace
negotiations. Yōji Itami is a JSDF officer sent to investigate the
other world, where magic, dragons and elves are real, using his
knowledge of fantasy stories to make his way in this new
environment.