In the 17th century Grotius argued that States exist in a loose community, but there may be enough of an international community to generate duties of mutual aid. In the 18th century Vattel argued that States clearly owe each other assistance because they are all part of a larger community of States. Today, we also speak of an international community, as when it is said that crimes against peace or crimes against humanity harm the international community. In those historical and contemporary cases, theorists have sometimes tried to defend such claims by reference to the idea of solidarity among States. I will argue that solidarity is importantly associated with minimal duties of mutual aid within that community. But solidarity has too many conceptual and normative problems to be a strong grounding for such duties. Nonetheless I employ the idea of solidarity to defend a minimal duty of mutual aid of States that is able to justify some wars, although not as robust a duty as many human rights theorists seem to want today.

The duty of mutual aid of States is the counterpoint to the general prohibition on war that pacifists and even Just War theorists have defended over the centuries.