Rumble Garanndoll

Japan in 2019, right before a new era begins. An alternate universe called "Shinkoku Nippon" with the sky and the earth upside down is suddenly revealed as a fissure to another realm suddenly appears in the sky. This alternate reality maintains the militarism of its previous age, which is known as Eternal Showa. The military uses enormous humanoid robots known as "Garann" and their gas weapons "Genmu" to invade the actual Japan, rendering our advanced armaments useless. "Shinkoku Nippon" immediately takes control of the government and effectively conquers Japan. For Japan, the "Reiwa" era has not yet begun.