So what exactly happened to Earth?

I'm still trying to understand how GLT's world ended up like that. It's pretty obvious that a war happened, but in the series there are both WW2 weapons and futuristic technology. Why? When did this war happen? Who's grandpa? Where were Chito and Yuuri before starting their journey? What happened to those people? They can't be ALL dead, right? Is the world really over, or only the big city where Chito and Yuuri live? Because in episode 12, people in the footages talk about "great victory over the *neighboring states*". Maybe somewhere else in the world there's still humanity?

Everything about their world is just so...vague. Are these fair questions, or am i the one who's watching GLT in a too rational way?