It happened with our brilliant and belated realization that there are vehicles out there fully capable of running on alcohol, which has been a huge boon to the people of New Haven and beyond. Fuel is what keeps our society running as we work toward our goals.
But on the larger scale, the society itself runs on a different kind of fuel: people. In the normal run of things human beings are much harder to come by than simple supplies. We can always make more of them, but it takes time and love and effort, and in the world we live in now there are enough dangers to make that a so-so prospect at best.
Which is why the UAS was clever. They knew they'd have to work something out to effectively wage the war they're pushing. Even now they've deeply engaged Ketill and his people in the west, taking risks and being aggressive--almost wasteful--with the people who fill their ranks. Why would the UAS be so casually warlike? Because they know a steady stream of fresh recruits waits for the chance at a better life.
They're using South Americans.
The southern hemisphere has been mostly forgotten and ignored since The Fall began, which is certainly a shame but does not make that part of the world especially unique. Zombies popped up everywhere after the initial outbreak in Cincinnati, and no one got off easy. I admit to being particularly blind to events south of the border, but the UAS is perfectly positioned to use the desperation of survivors trying to flee the region for their own purposes.
Unfortunately those survivors bring the undead with them. That's where the massive swarms in the south are coming from. We have to assume that whatever the UAS did to restore communications and land travel between the north and central/south America, it also opened the floodgates for the hundreds of thousands--possibly millions--of zombies chasing the moving survivors from there to here.
Which means they're now our problem. It feels better to know the facts even if there's nothing we can do about them at present. New Haven is already prepping to send fighters to the south in a concentrated effort to lay waste to the undead. If this is a deliberate tactic on the part of the UAS, it's effective; we'll have to focus on fighting the dead instead of waging war on the living. The UAS might achieve their goals without ever firing a shot. We can handle a lot of zombies, but fresh herds in the hundreds of thousands? No community can stand against that.
But by God, we'll try.