The world Ella's from could basically be considered all but dead. Roughly five hundred years ago, the Armored Cores used a mysterious energy source called 'Kojima Particles' that granted them immense energy. These ACs were called 'NEXTs', and while their combat abilities were insanely superior to the normal ACs, they had an unforeseen side effect. Every battle caused more and more of these particles to build up in the environment, eventually turning most of the planet into an unlivable toxic nightmare. Humanity only survived by abandoning the NEXTs and living in the few areas that were hospitable.