The Catholic Church's teaching on the principle of double effect and just war theory provides a nuanced framework for evaluating the moral legitimacy of the use of military force.

The principle of double effect states that an action which causes a serious harm, such as the death of human beings, may nevertheless be morally permissible if the action meets certain strict conditions.1 Specifically, the action must not be intended to cause the harm, the harm must not be the means by which the good effect is obtained, and the good effect must outweigh the harm.1

The just war theory, as outlined in the Catechism of the Catholic Church, lays out the rigorous conditions that must be met for the use of military force to be morally legitimate.2 These include:

1. The damage inflicted by the aggressor must be "lasting, grave, and certain."2
2. All other means of resolving the conflict must have been shown to be "impractical or ineffective."2
3. There must be a "serious prospect of success."2
4. The use of arms must not "produce evils and disorders graver than the evil to be eliminated."2

The Church acknowledges that in the modern age of weapons of mass destruction, it has become "very difficult" to meet these strict criteria for a "just war."3 As Pope Francis has stated, "Never again war!"3

At the same time, the Church recognizes the legitimate right and obligation of nations to defend human life and the common good against grave threats such as terrorism and persecution.4 But this must be done with great moral restraint, respecting ethical limits on the use of force and focusing on addressing the root causes of conflict.4

In summary, the Catholic Church's teaching on the principle of double effect and just war theory provides a framework for evaluating the moral legitimacy of the use of military force. However, the Church also emphasizes the immense moral gravity of such decisions and the difficulty of meeting the strict criteria in the modern age, calling for a renewed commitment to peace and nonviolent conflict resolution.