1.I think it was jp wn and dude had reached a magical world with an ability to summon armies of earth starting from WW2 to the modern era. He makes his armed forces and island country named as parabellum or something and is trying to kill off some other isekaiers . It also has a bunch of cliche bullshit yandere female subordinates and all that.
Maybe this based on the summary? I dunno. I just put in your criteria into the series finder... Modern Weapons Cheat in Another World