The surface of the world is diversified by certain features--rivers, chains of hills, deserts, marshes, seas, etc.--the passage across which presents difficulties peculiar to an army, and it is essential to the reading of military history to appreciate these difficulties; for the degrees of impediment which natural features present to thousands upon the march are utterly different from those which they present to individuals or to civilian parties in time of peace. Since it is to difficulties of this latter sort that we are most accustomed by our experience, the student of a campaign will often ask himself (if he is new to his subject) why such and such an apparently insignificant stream or narrow river, such and such a range of hills over which he has walked on some holiday without the least embarrassment, have been treated by the great captains as obstacles of the first moment. [Illustration: Map showing the peril of Marlborough's march to the Danube, beyond the hills which separate the Rhine from the Danube.] The reason that obstacles of any sort present the difficulty they do to an army, and present it in the high degree which military history discovers, is twofold. First, an army consists in a great body of human beings, artificially gathered together under conditions which do not permit of men supplying their own wants by agriculture or other forms of labour. They are gathered together for the principal purpose of fighting. They must be fed; they must be provided with ammunition, usually with shelter and with firing, and if possible with remounts for their cavalry; reinforcements for every branch of their service must be able to reach them along known and friendly (or well-defended) roads, called their _lines of communication_. These must proceed from some _base_, that is from some secure place in which stores of men and material can be accumulated. Next, it is important to notice that variations in speed between two opposed forces will nearly always put the slow at a disadvantage in the face of the more prompt. For just as in boxing the quicker man can stop one blow and get another in where the slower man would fail, or just as in football the faster runner can head off the man with the ball, so in war superior mobility is a fixed factor of advantage--but a factor far more serious than it is in any game. The force which moves most quickly can "walk round" its opponent, can choose its field for action, can strike in flank, can escape, can effect a junction where the slower force would fail. It is these two causes, then--the artificial character of an army, with its vast numbers collected in one place and dependent for existence upon the labour of others and the supreme importance of rapidity--which between them render obstacles that seem indifferent to a civilian in time of peace so formidable to a General upon the march.