LotR -- Book 5 - Chapter 04 - The Siege of Gondor

[Kuruharan]

I'm not saying the success of his mission depended on it, I'm saying it had become a part of it. It wasn't just a matter of just defeating Sauron, & then walking away from the possible chaos that might result, it was about leaving Middle-earth in some kind of order for its new beginning. Having Aragorn become king, re-establishing the Royal line, etc, were all part of that.

This may be deserving of its own topic, but...

I'm not sure that Middle-earth would have been left in chaos. Gondor would have been left under the rule of Faramir (and I can think of far worse people to be ruled by). Rohan might have ended up being ruled by Eowyn. That opens up the possibility of a Gondor/Rohan merger...

I guess the real difference is that there would have been no king over the wide empty spaces of Eriador.