At a time when some informed Muslims believe Islam to be “set on a collision course with the West” and some Christians warn of a “perhaps irrational but surely historical reaction of an ancient rival against our Judeo-Christian heritage,” it may be helpful to ask what both traditions teach about current issues of joint concern. One of the most urgent of these issues is the question of forcible humanitarian intervention. Setting aside broader questions of interpretation, such as the extent to which Islam can be identified with political radicalism and Christianity with Western political interests, this essay compares Muslim and Christian teachings on this issue. Both traditions have had to confront questions of political violence, suffering, and war since at least the time of the conversion of Constantine in one case (1,600 years ago) and the founding of the first Islamic state at Medina in the other (1,300 years ago). The central argument in this essay is that there is a surprising measure of agreement between the two traditions on the question of forcible humanitarian intervention, enough to provide the basis for a shared doctrine.