There is a danger of war in Europe to-day; in spite of peace pacts, conferences and other attempts to establish security one nation does not trust another, and war may arise at any moment. On the other hand, it may never arise, and upright men of all nations are endeavouring to prevent it by every means within their power. In all this Catholics are vitally concerned, for we form a large section of the European community, and are directly concerned about the welfare of that community. We must not be blind to obvious facts, we must know what is expected of us if war should actually break out; at the same time, we must show a practical desire for peace in Europe, and assist in any way possible to secure it. But our actions must be guided by sound principles, clearly understood and appreciated. Without such clear intellectual judgment we may be carried away by sentiment either to glorify war in militarist fashion, or to condemn war absolutely as something of its nature evil, like murder or adultery. Either view might lead to the worst disasters; neither is a view which a Catholic may hold. Popes and Bishops have repeatedly insisted on the evils of war, but they nave never categorically asserted that war is always and inevitably sinful. Rather they have pointed out that war may sometimes be just. Living in the midst of a world crying out for peace we may find this attitude of the Bishops rather disconcerting, and the reasons for it are not always clear. On these reasons, on the principles behind the teaching of Pope and Bishops, the present writer has attempted to shed a little light.