This is a book about naval strategy. It is therefore of a semi-technical nature, which may seem at first sight to limit its appeal to the technical reader. I make no apology, however, for addressing myself to the layman as well as to the professional officer. It used to be thought at one time that war was the affair of the fighting forces only and that fighting men were the only ones who were competent to express an opinion on matters of strategy. The late war showed the fallacy of those ideas. It showed that modern war is an affair of whole nations and not merely of armies and navies. It also made it clear that the final responsibility for strategy lay with the civilian government. Indeed, as the war progressed, the War Cabinet found itself taking a more and more searching interest in the determination of strategy. Under these conditions, where the civilian representatives of the public play an active part in the framing of strategy, it is most desirable that the public itself should have a working knowledge of strategical principles. Though the Government may often be forced by the exigencies of the case to come to vital decisions concerning the conduct of the war without previously taking the public into its confidence, there can be no doubt that it will be greatly strengthened in making those decisions if it can feel that it has behind it an instructed public opinion on strategical matters; a public opinion which is capable of forming a just and reliable estimate of the soundness or otherwise of the strategy adopted, as it is seen to develop. In a world, therefore, where warfare is not only a possibility but seems at the moment a matter of increasing probability, it appears to be the duty of every citizen to acquaint himself with the main outlines of strategy.