Our third elementary principle is the idea of concentration of effort, and the third characteristic of naval warfare which clashes with it is that over and above the duty of winning battles, fleets are charged with the duty of protecting commerce. In land warfare, at least since laying waste an undefended part of your enemy's country ceased to be a recognised strategical operation, there is no corresponding deflection of purely military operations. It is idle for purists to tell us that the deflection of commerce protection should not be permitted to turn us from our main purpose. We have to do with the hard facts of war, and experience tells us that for economic reasons alone, apart from the pressure of public opinion, no one has ever found it possible to ignore the deflection entirely. So vital indeed is financial vigour in war, that more often than not the maintenance of the flow of trade has been felt as a paramount consideration. Even in the best days of our Dutch wars, when the whole plan was based on ignoring the enemy's commerce as an objective, we found ourselves at times forced to protect our own trade with seriously disturbing results. Nor is it more profitable to declare that the only sound way to protect your commerce is to destroy the enemy's fleet. As an enunciation of a principle it is a truism--no one would dispute it. As a canon of practical strategy, it is untrue; for here our first deflection again asserts itself. What are you to do if the enemy refuses to permit you to destroy his fleets? You cannot leave your trade exposed to squadronal or cruiser raids while you await your opportunity, and the more you concentrate your force and efforts to secure the desired decision, the more you will expose your trade to sporadic attack. The result is that you are not always free to adopt the plan which is best calculated to bring your enemy to a decision. You may find yourself compelled to occupy, not the best positions, but those which will give a fair chance of getting contact in favourable conditions, and at the same time afford reasonable cover for your trade. Hence the maxim that the enemy's coast should be our frontier. It is not a purely military maxim like that for seeking out the enemy's fleet, though the two are often used as though they were interchangeable. Our usual positions on the enemy's coast were dictated quite as much by the exigencies of commerce protection as by primary strategical reasons. To maintain a rigorous watch close off the enemy's ports was never the likeliest way to bring him to decisive action--we have Nelson's well-known declaration on the point--but it was the best way, and often the only way, to keep the sea clear for the passage of our own trade and for the operations of our cruisers against that of the enemy.