Rousseau, in his 'Of the Social Contract', chapter IV, 'Slavery', puts that:
Men, from the mere fact that, while they are living in their primitive independence, they have no mutual relations stable enough to constitute either the state of peace or the state of war, cannot be naturally enemies. War is constituted by a relation between things, and not between persons; and, as the state of war cannot arise out of simple personal relations, private war, or war of man with man, can exist neither in the state of nature, where there is no constant property, nor in the social state, where everything is under the authority of the laws.
Well, to me this seems a rather obscure definition of what war is. I can't see how a relation is necessary for war to exist. To me it seems likely that any conflict of interest, even between previously unrelated entities, is prone to generate war - e.g two previously unrelated princes interested in the same territory.
I can find his logic behind there not being a possibility of war between two persons, by assuming a conflict between both would be a fight and not a war, but that is as far as I got in following his logic. But what he means by "real relations" and how these relate to the legitimacy of war, within his logic, that's far from being clear to me.