The attack and defence of oversea expeditions are governed in a large measure by the principles of attack and defence of trade. In both cases it is a question of control of communications, and in a general way it may be said, if we control them for the one purpose, we control them for the other. But with combined expeditions freedom of passage is not the only consideration. The duties of the fleet do not end with the protection of the troops during transit, as in the case of convoys, unless indeed, as with convoys, the destination is a friendly country. In the normal case of a hostile destination, where resistance is to be expected from the commencement of the operations, the fleet is charged with further duties of a most exacting kind. They may be described generally as duties of support, and it is the intrusion of these duties which distinguish the naval arrangements for combined operations most sharply from those for the protection of trade. Except for this consideration there need be no difference in the method of defence. In each case the strength required would be measured by the dangers of interference in transit. But as it is, that standard will not serve for combined expeditions; for however small those risks, the protective arrangements must be sufficiently extensive to include arrangements for support. Before dealing with this, the most complex aspect of the question, it will be well to dismiss attack. From the strategical point of view its principles differ not at all from those already laid down for active resistance of invasion. Whether the expedition that threatens us be small or of invasion strength, the cardinal rule has always been that the transports and not the escort must be the primary objective of the fleet. The escort, according to the old practice, must be turned or contained, but never treated as a primary objective unless both turning and containing prove to be impracticable. It is needless to repeat the words of the old masters in which this principle lies embalmed. It is seldom that we find a rule of naval strategy laid down in precise technical terms, but this one is an exception. In the old squadronal instructions, "The transports of the enemy are to be your principal object," became something like a common form. Nor did this rule apply only to cases where the transports were protected by a mere escort. It held good even in the exceptional cases where the military force was accompanied or guarded by the whole available battle strength of the enemy. We have seen how in 1744 Norris was prepared to follow the French transports if necessary with his whole force, and how in 1798 Nelson organised his fleet in such a way as to contain rather than destroy the enemy's battle-squadron, so that he might provide for an overwhelming attack upon the transports.