Beside this fundamental principle we must place another that is scarcely less important. Owing to the general common nature of sea communications, attack and defence of trade are so intimately connected that the one operation is almost indistinguishable from the other. Both ideas are satisfied by occupying the common communications. The strongest form of attack is the occupation of the enemy's terminals, and the establishment of a commercial blockade of the ports they contain. But as this operation usually requires the blockade of an adjacent naval port, it also constitutes, as a rule, a defensive disposition for our own trade, even when the enemy's terminal area does not overlap one of our own. In the occupation of focal areas the two ideas are even more inseparable, since most, if not all, such areas are on lines of communication that are common. It will suffice, therefore, to deal with the general aspect of the subject from the point of view of defence.
It was in conformity with the distinction between fertile and infertile areas that our old system of trade defence was developed. Broadly speaking, that system was to hold the terminals in strength, and in important cases the focal points as well. By means of a battle-squadron with a full complement of cruisers they were constituted defended areas, or "tracts" as the old term was, and the trade was regarded as safe when it entered them. The intervening trade-routes were left as a rule undefended. Thus our home terminals were held by two battle-squadrons, the Western Squadron at the mouth of the Channel, and the North Sea or Eastern Squadron with its headquarters usually in the Downs. To these was added a cruiser squadron on the Irish station based at Cork, which was sometimes subordinate to the Western Squadron and sometimes an independent organisation. The area of the Western Squadron in the French wars extended, as we have seen, over the whole Bay of Biscay, with the double function, so far as commerce was concerned, of preventing the issue of raiding squadrons from the enemy's ports, and acting offensively against his Atlantic trade. That of the North Sea squadron extended to the mouth of the Baltic and the north-about passage. Its main function during the great naval coalitions against us was to check the operations of Dutch squadrons or to prevent the intrusion of French ones north-about against our Baltic trade. Like the Western Squadron, it threw out divisions usually located at Yarmouth and Leith for the protection of our coastwise trade from privateers and sporadic cruisers acting from ports within the defended area. Similarly, between the Downs and the Western Squadron was usually one or more smaller squadrons, mainly cruisers, and generally located about Havre and the Channel Islands, which served the same purpose for the Norman and North Breton ports. To complete the system there were flotilla patrols acting under the port admirals and doing their best to police the routes of the coastwise and local traffic, which then had an importance long since lost. The home system of course differed at different times, but it was always on these general lines. The naval defence was supplemented by defended ports of refuge, the principal ones being on the coast of Ireland to shelter the ocean trade, but others in great numbers were provided within the defended areas against the operations of privateers, and the ruins of batteries all round the British shores testify how complete was the organisation.