Take, first, the value of cavalry in modern battle. In his April Despatch, Sir Douglas Haig enters on a strong defence of it--the plea of a great cavalry leader. Since the stabilisation of the trench system in the West, it has been, as we can all remember, a commonplace of the newspapers and of private conversation that cavalry were played out--a mere useless or ornamental excrescence on armies that, by the help of tanks and aeroplanes, could now excellently do without them. "Not at all," replies Sir Douglas Haig. If the German Command had had at their disposal last March and April "even two or three well-trained cavalry divisions, a wedge might have been driven between the French and British armies." In any case, the difficulties of our task would have been greatly increased. On the other hand, our cavalry were enormously useful to us in the same battle. "So great indeed became the need for mounted men that certain units which had been dismounted were hurriedly provided with horses and did splendid service. Frequently when it was impossible to move forward other troops in time our mounted troops were able to fill gaps in our line and restore the situation." During the long trench battle of the middle years "the absence of room for manoeuvre made the importance of cavalry less apparent." But in the last stage of the struggle, when the Germans "were falling back in disorganised masses," the moral effect of British cavalry pressing on the heels of the enemy was "overwhelming," and had not the Armistice stopped the cavalry advance, it would have turned the enemy's disorganised retreat "into a rout." This is strong testimony, and will probably be stoutly fought by the eager advocates of "mechanical contrivances." But Sir Douglas Haig stands to it that no form of mechanical contrivance can ever either make the cavalryman useless, or the infantryman, who is "the backbone of defence and the spearhead of attack," less important. He admits, indeed, fully that machine guns, tanks, aeroplanes, and motor transport "have given a greater driving power to war," and that the country which possesses most of such things has an advantage over its opponents. But he insists that their only "real function" is to assist the infantry to get to grips with their opponents, and that of themselves "they cannot possibly obtain a decision." To imagine that tanks and aeroplanes can ever take the place of infantry and cavalry is to do these marvellous tools themselves a disservice by expecting of them more than they can perform. "Only by the rifle and bayonet of the infantry can the decisive victory be won." For, as the Commander-in-Chief lays down no less strongly than this great French colleague, Marshal Foch, "this war has given no new principles." But it has greatly complicated the application of the old. Every new invention makes the problem of co-operation--of interaction between the different armies and services--more difficult and more imperative.