Remember "we'll turn the food production down to force them to go to war to loot for food, otherwise, they will die"?
Wasn't my call. Obviously that was never going to work, because without food, you can't sustain anything for more than a couple of days.
I'm not really aware of all the details of your intents, though, so maybe I'm wrong. Maybe I'm way off. But there is still a good chance that, indeed, they WILL die, and then they'll come whine about it.
I'm absolutely sure this is what some of them will do. And when they do, I will relate to them the story of the man in the flood who ignored the boats and helicopter sent to save him.
I'd like to say that even a sudden targetting of half the continent wouldn't need instantly killing the realms without giving the players a chance to move elsewhere. The changes could be gradual in effect, instead of in geography. You could take up half of the continent, and then slowly apply things like dwindling production and shortage of materials (no building).
That's actually much harder to calibrate the right speed for, because if you go over for just a little while, bam! everyone's dead already. If we push the glacier a little too fast for a while, well, you'll have lost some more regions, but the ones you have will still be in as good shape as they were before.
