Chapter 1 of the Gate - Thus the JSDF Fought There! manga is titled, "Beginning of War With Another World".
In the summer of 20XX, a mysterious gate appeared in Tokyo City Central, Ginza at 11:50 AM. When the gate opened hundreds of monsters, dragons, and legions of soldiers appeared, wreaking havoc upon the city.
Itami Yōji, for his own selfish reasons, defended the Japanese civilians by evacuating them into the imperial palace. The JSDF later arrived and pushed the invaders back into the gate with the combined forces of the Tokyo Police.
In response to this surprised invasion, the Japanese prime minister pushed forward his policy to deploy the JSDF into the Gate to the place now called the Special Region. At the Empire's capital, the Senate was debating over the failure of the invasion of the Gate the number of deaths the action had actually caused.
Marquis Casel was particular vocal over the discussion. Despite knowing the situation, Emperor Molt Sol Augustus was not deterred and ordered an assault on Alnus hill. Three waves of assault had been staged and resulted in massive casualties and the elimination of the Allied army.
Looking over the corpse ridden battlefield, Itami Yōji wondered just what sort of a country would send that many men to die and even thought that it might be sign of a country nearing its end before heading back to base camp. At the base, his CO then appointed him the commander of the 3rd Recon Team of which his mission was to scout out the region and establish a friendly contact with the locals.