[Reboot version] Four Worlds Assault the Isekai Empire

Once an empire that was foul enough to try and successfully open a multiple portal/gate to another world called "Earth" but because of something, It led to 4 eras of them which seem to be in different universes. After the first attack, Zalgrerent will now pay the price of attacking our world, and those nations from 4 different eras are coming for them. "Show them no mercy and fire at will, Kill or be killed." === This is a reboot version of "Four Worlds Confront the Isekai Empire" This version will be realistic and more historically/militarily authentic. All information about military equipment will be researched deep down to the point I can't find anything from it anymore. We hope you enjoy this story.