Hence the importance of realising the distinction between the two generic forms of naval activity. In the hurry and stress of war confusion between them is easy. By keeping a firm grip upon the difference we can see at least what we are doing. We can judge how far any given operation that may be called for is a sacrifice of security to exercise, how far such a sacrifice may be justified, and how far the one end may be made to serve the other. By applying the distinction as a test much error may be avoided. The risk we take may be great, but we shall be able to weigh it accurately against the value of the end, and we shall take it with our eyes open and of set purpose. Above all, it will enable the Staff to settle clearly for each squadronal commander what is to be his primary objective, and what the object or purpose of the operations entrusted to him. It is above all in this last consideration, and particularly in the determination of the objective, that lies the main practical value of the distinction. This will become clear the moment we begin to consider defence against invasion, which naturally takes the first place amongst operations for the exercise of control. Of all the current assumptions, not one is so confusing for the finer adjustments of strategy as that which affirms that the primary objective of our fleet is always the enemy's fleet. Of the battle-fleet and its attendant units it is of course true, so long at least as the enemy has a battle-fleet in being. It is true, that is, of all operations for securing control, but of operations for exercising control it is not true. In the case we have now to consider-defence against invasion-the objective of the special operations is, and always has been, the enemy's army. On this fundamental postulate our plans for resisting invasion have always been constructed from the year of the Armada to 1805.