That's a notoriously difficult question. For what it's worth, I think justice is an emergent property of a well-functioning society, but that's not important right now.
It is not the same thing as fairness. Fairness is a more limited but less ambiguous concept, resting on equality of treatment. If there's no good reason to prefer A over B, then A and B should be treated the same.
If A and B have a dispute, the fair thing is to split the disputed entity evenly, or to toss a coin. That may not be the just thing however — but justice is difficult and might depend on all the details of the dispute.
(Fairness can extend a bit further than that. If A and B made an agreement, and A has complied with it, then B should too, even if the agreement imposed different demands on each of them. It is not fair for the agreement to be enforced on one party but not the other).
Games and sports, in particular, should be fair. The reason we want them to be fair, is that it makes the result less predictable, which is more exciting. People will neither play or watch sports where the outcome is not in doubt. And the authors of the sport's rules want people to play the sport.
War is the same. If it is made fair, then people will be more willing to play. There is a difference, though, which is that in general we do not want to encourage people to play war.
Which takes me finally to this tweet from "end of tyranny":
#NFZ levels the battle field, which ain't in #Qaddafi's favor. Here's to a free #LibyaThe level battlefield. The only thing that nobody should want.
There are three reasonable positions one could have toward the conflict in Libya. One could want Gadaffi to win. One could want the opposition to win. Or one could want peace.
A "level battle field" is not a means to any of those ends. It is a means only to encouraging war for its own sake. To create it on humanitarian grounds is insane.
I think Aretae makes a similar, if less blatant, error in the post I discussed earlier.
He says, in the context of politics:
Manipulating the rules of the game has a high likelihood of having SUBSTANTIALLY higher returns than competing on a fair playing fieldPolitics, like war to which it is closely related, does not take place on a playing field. Making politics more fair will not necessarily make the outcome more just, but will make participation more attractive, which is a bad thing.