Excerpts from: Compendium of the Social Doctrine of the Church, Pontifical Council for Justice and Peace, 2004
502. The requirements of legitimate defence justify the existence in States of armed forces, the activity of which should be at the service of peace. Those who defend the security and freedom of a country, in such a spirit, make an authentic contribution to peace.  Everyone who serves in the armed forces is concretely called to defend good, truth and justice in the world. Many are those who, in such circumstances, have sacrificed their lives for these values and in defence of innocent lives. Very significant in this regard is the increasing number of military personnel serving in multinational forces on humanitarian or peace-keeping missions promoted by the United Nations. 
503. Every member of the armed forces is morally obliged to resist orders that call for perpetrating crimes against the law of nations and the universal principles of this law. Military personnel remain fully responsible for the acts they commit in violation of the rights of individuals and peoples, or of the norms of international humanitarian law. Such acts cannot be justified by claiming obedience to the orders of superiors.
Conscientious objectors who, out of principle, refuse military service in those cases where it is obligatory because their conscience rejects any kind of recourse to the use of force or because they are opposed to the participation in a particular conflict, must be open to accepting alternative forms of service. "It seems just that laws should make humane provision for the case of conscientious objectors who refuse to carry arms, provided they accept some other form of community service".