Chapter 1 of the Gate - Thus the JSDF Fought There! manga is titled, "Beginning of War With Another World".
Plot
In the summer of 20XX, a mysterious gate appears in Tokyo City Central, Ginza at 11:50 AM. When the gate opens, hundreds of monsters, dragons, and legions of soldiers appear, wreaking havoc upon the city.
Itami Yōji, for his own selfish reasons, defends the Japanese civilians by evacuating them into the imperial palace. The JSDF later arrives and pushes the invaders back into the gate with the combined forces of the Tokyo Police.
In response to this surprised invasion, the Japanese prime minister pushes forward his policy to deploy the JSDF into the Gate, forming a place and naming it the Special Region. At the Empire's capital, the Senate is debating over the failure of the invasion of the Gate the number of deaths the action has actually caused.
Marquis Casel is particularly vocal over the discussion. Despite knowing the situation, Emperor Molt Sol Augustus is not deterred and orders an assault on Alnus hill, where the JSDF has setup. Three waves of assault are staged and result in massive casualties and the elimination of the Empire's army.
Looking over the corpse ridden battlefield, Itami Yōji wonders 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 appoints him the commander of the 3rd Recon Team of which his mission is to scout out the region and establish a friendly contact with the locals.