The Philosophy of Law: Public Right, § 56: Right of Going to War in Relation to Hostile States
Viewed as in the state of Nature, the Right of Nations to go to War and to carry on hostilities is the legitimate way by which they prosecute their Rights by their own power when they regard themselves as injured; and this is done because in that state the method of a juridical Process, although the only one proper to settle such disputes, cannot be adopted.
The threatening of War is to be distinguished from the active injury of a first Aggression, which again is distinguished from the general outbreak of Hostilities. A threat or menace may be given by the active preparation of Armaments, upon which a Right of Prevention (jus prccventionis) is founded on the other side, or merely by the formidable increase of the power of another State (potestas tremenda) by acquisition of Territory. Lesion of a less powerful country may be involved merely in the condition of a more powerful neighbour prior to any action at all; and in the State of Nature an attack under such circumstances would be warrantable. This international relation is the foundation of the Right of Equilibrium, or of the ‘balance of Power/ among all the States that are in active contiguity to each other.
The Right to go to War is constituted by any overt act of Injury. This includes any arbitrary Retaliation or act of Reprisal (retorsio) as a satisfaction taken by one people for an offence committed by another, without any attempt being made to obtain reparation in a peace ful way. Such an act of retaliation would be similar in kind to an outbreak of hostilities without a previous Declaration of War. For if there is to be any Right at all during the state of war, something analogous to a Contract must be assumed, involving acceptance on the one side of the declaration on the other, and amounting to the fact that they both will to seek their Right in this way.