Concentration of this nature, moreover, will be the expression of a war plan which, while solidly based on an ultimate central mass, still preserves the faculty of delivering or meeting minor attacks in any direction. It will permit us to exercise control of the sea while we await and work for the opportunity of a decision which shall permanently secure control, and it will permit this without prejudicing our ability of bringing the utmost force to bear when the moment for the decision arrives. Concentration, in fact, implies a continual conflict between cohesion and reach, and for practical purposes it is the right adjustment of those two tensions--ever shifting in force--which constitutes the greater part of practical strategy. In naval warfare this concentration stage has a peculiar significance in the development of a campaign, and at sea it is more clearly detached than ashore. Owing to the vast size of modern armies, and the restricted nature of their lines of movement, no less than their lower intrinsic mobility as compared with fleets, the processes of assembly, concentration, and forming the battle mass tend to grade into one another without any demarcation of practical value. An army frequently reaches the stage of strategic deployment direct from the mobilisation bases of its units, and on famous occasions its only real concentration has taken place on the battlefield. In Continental warfare, then, there is less difficulty in using the term to cover all three processes. Their tendency is always to overlap. But at sea, where communications are free and unrestricted by obstacles, and where mobility is high, they are susceptible of sharper differentiation. The normal course is for a fleet to assemble at a naval port; thence by a distinct movement it proceeds to the strategical centre and reaches out in divisions as required. The concentration about that centre may be very far from a mass, and the final formation of the mass will bear no resemblance to either of the previous movements, and will be quite distinct.