Here's what happened. Five years ago, ayys arrived and fucked everything over, the end.

If you really need to know how they fucked everything over (You don't), then read this shit.

It started, as any regular story does, with strange lights and mass murder. Specifically, the sudden genocide of absolutely fucking everything above the 60° parallel North and the 50° parallel South at the hands of the Arume.
They were all killed overnight, but the world was largely left alone. Of course, the Arume had royally pissed off both the Russian, American and, to a lesser degree, Chinese governments by murdering a decent chunk of their populace (In China's case it was just a bunch of Antarctic scientists, really, but that didn't stop them from violently sperging out). Then, after throwing a couple of nukes into the poles and getting nuked right back by the bunch of fucking anime girls that had taken residence up in the most inhospitable corners of the planet, everything just fell apart.
The Arume really only cared about keeping the poles for whatever reason (It sure as fuck wasn't because they're a pleasant place to live - They're just as resistant to cold as a regular human, perhaps even less), but in the end they judged that Earth's native population was simply too troublesome to keep around and they decided to take over. They could overrun humanity easily enough, but they simply didn't have the manpower to hold any of the land they took over efficiently.
The status quo for the last couple of years has been this: The aliens keep the more sparsely populated parts of the world firmly under their control. Other than that, they have a few bases spread here and there, a few small countries' worth of land that they've managed to stake their claim over and everything else is constantly getting taken by the resistance and retaken shortly afterwards.