My answer is: competition between societies.
And during most of human history, this competition took the form of warfare (this logic is explained in this post).
My analysis shares some common ground with that of Ian’s, and differs significantly in other ways. But I agree with him that it was war that created large cooperative societies. So when we abolish war, as I believe we must, we also need to be careful not to be hit by unintended consequences.
Suppose that Alpha Centaurians arrive in the Earth’s orbit, and effectively stop all interstate wars. Wouldn’t that be wonderful? Yes, for a while. But if the Centaurians do not implement an effective alternative mechanism for peaceful intersocietal competition, the utopia will gradually evolve into a dystopia.