How to give a satisfying ending to a sandbox campaign?

My next campaign is going to be an almost total sandbox - my players will join a crusade to regain territories taken by Orcs during the last crisis. These lands has belonged to Kniland, the kingdom the players came from, for over 1000 years and many people want to come back there, but Orcs have been living there for 40 years already and consider it their home, making the whole campaign morally gray.

The players are free to do whatever they find right and side with whoever they think is the good or least bad guys. They can retake territory, they can expand the human kingdom, they can side with orcs, they can try to work out compromises, they can abandon politics whatsoever and just work as adventurers, there's no one canon patch they need to take. I have prepared multiple factions, both inside the human and orc civilizations, and multiple terrains, so there's really nothing they cannot do.

Now, all of this sounds interesting, but how to end it well? If there's no a specific goal, there's also no specific place where the campaign will be over. How to end it in a satisfying way then?