Wars of conquest were less frequent in the Middle Ages than we might expect, and were usually waged on a small scale. Their comparative infrequency, in an age of militarism, must be explained by reference both to current morality and to economic conditions. For an attack upon a Christian power it was necessary that some just cause should be alleged. Public opinion, educated by the Church to regard Western Christendom as a single commonwealth, demanded that some respect should be shown to the ordinary moral code, even in international relations. Furthermore the medieval state, loosely knit together and bristling with isolated fortresses, showed in defeat the tenacious vitality of the lower organisms, and could not be entirely reduced without an expenditure, on the invader's part, which the methods of medieval state-finance were powerless to meet. Edward I failed to conquer the petty kingdom of Scotland; and the French provinces which were ceded to Edward III escaped from his grasp in a few years. The profitable wars were border wars, waged against the disunited tribes of Eastern Europe, or the decadent Moslem states of the Mediterranean. And such wars were of common occurrence, sometimes undertaken by the nationalities most favourably situated for the purpose, sometimes by self-expatriated emigrants in search of a new home. Thanks to the teaching of the Church, a large proportion of the border wars were converted into Crusades for the propagation of the faith or the extermination of the unbeliever or the defence of holy places. Often enough the religious motive was introduced as an afterthought, and gave a thin veil of respectability to operations which it would otherwise have been difficult to excuse. In some cases, however, those who enlisted as the soldiers of the Church were sacrificing their material interests for the good, as they supposed, of their own souls and the Christian commonwealth. There was nothing essentially Christian in this spirit of self-devotion; it had long been epidemic in the Mohammedan world, and accounts for the most successful encroachments of Islam upon Europe and the Eastern Empire. The impulse affected Western Christendom for a relatively short period of time, only once or twice producing movements at all commensurable with those which had emanated from Arabia, Asia Minor, and Africa, and leading to no conquests that can rank in magnitude with the caliphates of Bagdad, Cordova, and Cairo. But the Christian Crusade is in one sense more remarkable than the Mohammedan Jehad. Western Europe had long ago emerged from the nomadic stage, and even the ruling classes of Western Christendom, cosmopolitan as they may seem to us, were attached to their native soil by many ties. If the upheaval was smaller in the West than in the East, the material to be set in motion was more stubborn and inert, the prizes to be held before the eyes of the believer were more impalpable and dubious. There