Initial Imperial Incursion
In summer 20xx, a portal to another world appeared in Ginza, from which stormed a Roman-like army which included infantry and cavalry, as well as "fantasy" creatures such as dragons and goblins. The invading army begin to indiscriminately attack civilians, causing a few hundred deaths and taking an unknown number of civilians back through the Gate as slaves and war prisoners. The Imperials sustained a few casualties from the small arms and sub-machine gun carried by police officers on the scene but they eventually overwhelmed the officers due to their vast number.
Siege of the Japanese Imperial Palace
Yōji Itami, an off-duty JSDF lieutenant attending a doujinshi sale in Ginza intervened, taking command of the police force and evacuating civilians to the Japanese Imperial Palace, as well as personally killing one Imperial soldier in hand-to-hand combat. Itami and the police guarded the palace gates, arming themselves with tactical police equipment including handguns, shotguns, submachine guns, tear gas, and riot gear. The Imperial (as in the Empire) Army laid siege to the (Japanese) Imperial Palace by attempting to climb the walls using ladders, and employed battering rams and trebuchets against the palace gates and walls, but were held off by the sustained firepower of the police firearms.
Shortly after the initial invasion, JSDF ground forces, supported by helicopters and naval fire from an Asagiri-class destroyer, counterattacked against the Imperial army, decimating the enemy forces with far superior firepower. Over 60,000 Imperial troops are estimated to have been killed, and almost as many were taken as prisoners from the police force using water cannon, tear gas and anti-riot gears.
For his intervention, Itami was promoted to First Lieutenant, and later, the JSDF set up base and sent a Special Task Force into the Special Region to explore the new territory, eliminate any possible threats, and set up peace negotiations with the other world cultures.