As soon as it appeared, the Imperial Army invaded through the Gate and began to indiscriminately kill or capture every civilians they encountered, and overwhelming attempts by police forces to halt their advance failed. It was at that time that Yōji Itami took command of the Tokyo police in the area and led an evacuation of civilians behind the walls of the Japanese Imperial Palace. From the palace, the police held off the attackers until the Japan Self-Defense Forces arrived and decisively defeated them. The incident would become known as the Battle of Ginza.
After the incident, the Gate was sealed off with a dome-shaped structure and JSDF forces were deployed to the other side of the Gate, marking the start of JSDF counterattack. Following the Japanese victory at the First and Second Battle of Alnus Hill, the JSDF constructed a base centered around the Gate, which was later expanded to include a refugee camp as well, becoming the community of Alnus. It is later revealed that Hardy, the Goddess of the Underworld of the Special Region is responsible for opening the Gate in her whim in order to destroy the old order of the Empire by making them face an enemy far superior to them; the JSDF. Due to the fact that Gate only appears in Japan, it gives the country both huge advantages and disadvantages. The advantage of monopolizing the Gate enables Japan to have massive economic growth and influence in the global scale from accessing to massive untapped resources as well as a sole trader with locals in the Special Region. However, it is also putted Japan in the disadvantage position since the country now are being intimidated and pressured by a lot of superpowers like US, Russia, China or EU since these countries want a share of exploiting resources from the Special Region and trading with locals. The movement of the JSDF in mopping up the Imperial Troops during the early stage of the Japanese-Empire war and during the Imperial Civil War causing countries like Korea or Russia to accuse them as violating the treaty in WW2 about not creating an official army and becoming militarized. Despite the disadvantage, according to Akira Yaginada, Japan still has enough bargaining power against all countries in the Earth from their monopoly of the Gate. As the result, resources-producing countries like Russia, China, German as well as others countries put pressure on the UN to the petition that the Gate should be control by the UN so these countries can access to the Special Region.
The Gate phenomenon also affects the world stage in the Earth dramatically, beside the fact that all countries in the world want to have the share of exploiting the Special Region, the UN wants to be free from Russia's gasp since Russia is one of the world top natural resources producer and the Gate is surely can reduce their influence in the world stage due to the fact that once the EU and others country members could get their hands on a stable supply of resources from the Special Region, they would no longer need to heed Russia’s demand. Of course, if this happened, Russia would be in trouble, which was why they were demanding that the United Nations administer the Special Region. To Russia, it would have been best if the Gate had never existed, and they might even launch a SLBM to destroy the Gate and everything around it. However, Russia is not foolish to do that without escalating conflict with Japan, US and the EU.
After some times, the Gate randomly opens to the dimension of the Arachnid creating the devastating war for both the Empire and the JSDF until Lelei closes the gate with her magic to prevent more calamity. After Itami and some of the JSDF's personnel was trapped beyond the gate due to its closure, Lelei attempts to create a Gate with her magic to connect back to Japan. However, it is revealed that some superpower like US, Russia, and China are secretly researching in constructing a dimensional gate through quantum physics to reach the Special Region without having to rely on Japan's Gate. So far, they have little success since the technology necessary for the dimensional gate is far too advanced.
It is revealed that if the Gate opens for too long it cause the strain in the flow of space and time stream causing catastrophic event in two worlds. The prime example is earthquake in Special Region.
The gate was forced to close during the Arachnid War to prevent the Arachnid from devastating both Japan and the Special Region, stranding 4600 JSDF personnel.
After 4 years and before the events of Weight Anchor, Lelei successfully opens the Gate again using Itami's memories in which the Gate opens the second time in Japan but in Comicket, Akihabara. During the event of Weight Anchor, Japan once again monopolizes the new Gate to exploit resources from the Avion Sea. The reopening of the Gate also prompts the superpowers to create new plans to get the Gate for themselves once again.
The Gate is a portal that exists between many worlds and the Special Region. It periodically opens and closes, bringing new races and cultures to across. The Gate is thought to be supernatural in nature (related to the gods) and responsible for the incredible, almost implausible diversity of races existing within the Special Region. The Gate always opens on Alnus Hill, explaining why all the races consider it a Holy Land and the origin of their species within their respective mythologies.
According to Chapter 72 of the Manga, the Gate is 16 yards/meters across.
The Gate can happen randomly; being opened by a God, in this case Hardy or using magic in the case of Lelei. Furthermore, the Gate can be closed using magic or sealed through confined wall in the case when Gate threatens the time space continuum.
Despite being able to control the Gate, Gods in the Special Region have absolutely no jurisdiction in the world beyond the Gate like Japan since they are bounded by the planet in the Special Region and Earth is outside their reach. According to Rory, any JSDF members who die in the Special Region will be under her guidance to the afterlife while all died agents in the Hakone Incident were out of her capacity to send them to the afterlife since she was in Japan soil. For this issue, Rory vaguely states that Japan and the world beyond the Gate are belong to someone else's turf.
In case there are disruptions in the communications and logistics between Japan and Special Region or there are some phenomenons that can potentially harmful to the Japan because of the Gate, the Idaten protocol will be initiated to recall all 26 000 JSDF personnel to retreat back to Japan. There are two stages of the protocol: general retreat preparation and total retreat.
Firstly, for general retreat preparation, when the Idaten protocol is green-lighted, all JSDF personnel must prepare to pack their equipment to go back to Japan in some amount of days. Finally, as the total retreat order, codename White Rabbit, is executed, all personnel will immediately rush back to Japan as soon as possible. If the situation was dire enough, they would abandon the majority of military hardware back at Alnus. Since the vast majority of JSDF's weaponry utilized in the Special Region are archaic, it is not a significant loss.
In order to maintain Japanese's monopoly and administration of the Gate in the future, 4600 JSDF personnel like Yonji Itami, Tomita or Shino as well as diplomat like Sugawara will have to stay back in the Special Region until Lelei could create the Gate once again in the future.