In the framework of the issue of the observance of promises and agreements (De iure belli ac pacis, bk. ii), Grotius discusses the question of whether Christians should be allowed to conclude treaties or alliances (federa) with those who were named infideles in the canonical and theological terminology. The question was ancient: since the early Middle Ages, alliances of Christians with infidels had been labeled as ‘impious’ (impium fedus). Grotius’s solutions are based on the converging traditions of medieval canon law and theology: treaties and alliances with infidels are intrinsically lawful according to natural and positive divine law, although they should be avoided in certain circumstances. Grotius’s concern, however, was not so much to affirm the theoretical lawfulness of the fedus cum infidelibus. The outcome of his argument consists in promoting the cohesion of Christians: he thinks that a joint action of the Christian nations is a moral and juridical obligation in the face of the aggressions caused by the enemies of Christendom.