In the seventeenth 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 eighteenth 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 important and 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.2 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. In Just War theory the general prohibition on war was a contingent prohibition, and such prohibitions can sometimes be outweighed by other considerations. We here confront a problem that has plagued theorists since at least the time of Augustine: war is so horrible that it should generally be prohibited on moral grounds, but also sometimes necessary in order for us to meet our moral obligations of mutual aid. According to this construal, if war is to be justified it will be on grounds of defense of innocent others.I will focus on the crime of aggression, and on whether some acts of initiating war without provocation could be non-aggressive or at least justified on grounds of defense of others. So-called humanitarian war is the classic case of a war that seems t aggressive but which many people see as nonetheless justified if not required of States. I have previously defended contingent pacifism according to which any waging of war that kills innocent people is morally suspect.3 But in international criminal law, at least since the time of the Nuremberg trials, only waging aggressive war is seen as unjustified waging of war. What makes a war aggressive is an unprovoked attack by one State against another. I will investigate the difficult question of what the foundation is of a State's duty to aid another State, and why it is not unjustified for that State to engage in an unprovoked attack on a State that is itself attacking another State, or some of its own people.The article proceeds as follows. I begin with a discussion of Augustine and Vattel, surely the two most significant thinkers in the historical Just War tradition to have addressed our topic. In the second section, I examine the very idea of there being an international community in which there are legal duties. In the third section, I spend considerable time trying to explain what solidarity is and why it might be the key to understanding the duties of aid of the members of the international community. In the fourth section, I tackle some of the main objections to the idea of international solidarity. Finally, I discuss the idea that there are natural duties of justice that call for States to go to the aid of one another, especially when there are serious human rights abuses. Throughout, I discuss how the solidarity among people and among States might support peace and yet call for certain wars to be waged to sustain that peace.