The words just tumbled out of my mouth, and I regretted them as soon as I uttered them. I was GMing what looked like our final session of Wild Talents (transhuman, far future, sci fi version) for at least a while (and perhaps forever). These were the words of the Big Bad, the leader of the invading, universe hopping, barbarian, alien horde, and these guys were flat out stronger than the PCs. At least one of the PCs didn't like the position he was put in - he felt as if he were offered no choice and was essentially being railroaded to "join," and this is literally the last thing I want in a game. For a second, I thought about taking it all back, but hell, I was the GM, and I wouldn't allow a PC to do take backseys on an important decision made in the heat of the moment. In the end, a compromise was struck between the PCs and the Big Bad - the PCs joined, and the aliens would only occupy their universe for 5-10 thousand years on the way to domination of reality. In retrospect, I don't think my decision was that bad. Here are a couple factors to help you decide:
1) The PCs were more physically powerful than anything else in the game thus far, and flat out dominated (partially because of the version of the rules we were playing with). There were lots of diplomatic ramifications of the use of extreme force throughout the game, but the PCs could physically overcome anything until the alien horde. They knew it was coming, but didn't know how powerful it was.
2) The PCs largely ignored the horde, focusing instead on local diplomatic issues and immediate threats, until it hit them in the face. Part of our game setup was the impending threat of the horde, so it's not like they missed any hints.
3) We had intended to hack in Burning Wheel's social combat rules, because they're great (and work much better than their mechanically similar cousin rules for real combat, IMHO), but I forgot all about this until afterwards. That's a shame, because diplomacy was what won the day in the end (well, sort of won).
4) The PCs saw themselves getting torn apart by the horde in space, but proceeded straight to their leader without any subtlety. One PC (the one who complained) actually set a charge on his ship to blow up if things went to hell. He didn't use it. Let's just say I would've looked favorably upon interesting decisions besides fight or talk (you know, like escape or something creative). Also, one of the PCs could construct teleporters in a single round.
5) I certainly could've said something else besides "join us or die" that would give the PCs other options. Like: "Go back to your leaders and tell them my proposal. You have 5 hours, Earthlings. Don't be late. AHAHAHAHAHA!!!" But then this would've become more like a choose your own adventure instead of a rpg, right?
6) We definitely wanted to finish off this arc so Pat can run his west marshes game he's been talking about.
7) I really had no preconception about how all of this would turn out, and I actually had a lot more adventure planned for the evening. But it was all moot after this encounter, and it seemed like the obvious stopping point for the arc because it changed the game world so drastically.
Looking back after writing all this, it certainly seems to me like I didn't screw up. Though, I'm sure the PCs would add in 7 more points that bolster their side. At the very least, this seems like the flip side of Pat's stormtrooper post below (man, his low-level survival game is going to be a hell of a contrast to Wild Talents - I definitely think it'll work better than WT because I'm much more of an abstract spacey type than the other players...and most people I know).
Anyway, I'd sure appreciate any feedback our readers. You know, in the name of trying to become a better GM and all.