Since the later middle ages the Church has commonly taught that five main conditions must be fulfilled before a war may be entered into with justice and therefore graciously. These conditions are that the war has to be undertaken only as a last resort when all other methods of retaining or achieving peace have failed; that there are serious prospects of success; that the war must be waged only for a grave and just cause proportionate to the evils of war; that it must be declared and prosecuted only by the legitimate authority; and that the whole prosecution of the war be guided by the right intention. There is no need to discuss this tradition; it is sufficiently well-known and in certain circles has been discussed at such length as to become wholly complex and unreal. What is urgently needed is an objective view of the application of the principles involved in this tradition to the modem scene of international strife; in other words, we need to discover the full stream of Christian life and thought as it meets the ocean of the present at this point.

Tradition teaches—and we will accept it without argument— that war may be undertaken only as the last resort. As things exist today we can say that if and when a country suffers armed aggression and she is forced into a defensive war, then war is permissible under this heading because there is no alternative; but that international politics are so complicated that it is often extremely difficult to discover who is the real aggressor.