In all but the exceptional cases where neutral status is internationally defined or recognised, how we define ‘neutrality’ depends on how we define ‘war’ and arguments about that fall, broadly, into two categories. If we regard ‘war’ as a violation of some kind of system which gives value to and constrains the behaviour of the constituent states, such as a res publica Christiana or a League of Nations, then war is, in some sense, a criminal act against the community of states and ‘neutrality’ is logically excluded if one of its members flouts the rules. Justice demands that one cannot be neutral towards crime. If, however, we regard ‘war’ as a natural recourse of states in the pursuit of their objectives, then ‘neutrality’ is, equally, a legitimate option. It does not discriminate between ‘just’ wars and their opposite. War is war, and staying out is a right. But if that right is absolute, the resultant practice is highly contingent. A claim to neutrality formally obliges the neutral state to observe impartiality and abstention in its relationships with belligerents. Equally, it automatically confers rights on belligerents – specifically in regard to ‘un-neutral conduct’ arising from any failure to observe impartiality and abstention. This makes unilateral neutrality a precarious option. Its scope and limits were worked out in international law from the late seventeenth century, that is, after the era of religious conflicts in Europe and the effective abandonment of a civitas maxima constraining state behaviour.