British war policy was essentially non-aggressive. The Navy had but one possible antagonist of the first rank at sea, and that one we should never have fought with, except in a war of defence. Our submarines, therefore, had two obvious duties marked out for them. They would help in coast defence by making it dangerous for ships of war or transports to approach, and they might be used, if an opportunity arose, to attack a fleet in harbour, or a cruiser at sea. There was every probability that any fleet of a Power at war with us would sooner or later have to spend a good deal of time in port, and it would certainly be well to have the means to attack it there. But, important as this function was, the idea of defence against invasion probably came first, and there is no doubt that an efficient submarine force is a very formidable addition to our flotilla for coast defence. Perhaps we thought, in those years of perpetual preparation, too much about the ‘Invasion of England’ and too little about the duty of supporting our Allies on land; and we had this much justification, that the Power from which we had every reason to expect an attack, was one directed by men of great energy and determination, certain to be relentless in pressing a war home upon us, even at the risk of a heavy loss. On the other hand, those who spoke and wrote most about invasion, nearly always failed to realise the immense difficulty of the undertaking; and they failed especially to see that, in modern times, the conditions had changed very considerably in favour of the defence. The initial problem of an invader by sea must always be the provision of transport sufficient for a large body of troops, with arms, equipment, and supplies of food and munitions. Even if we allow only two tons of shipping per man--the Japanese allowed six tons--the transport of 100,000 men would take twenty vessels of 10,000 tons each, and to collect these and load them would be a big operation; difficult to conceal. In fact to conceal it, for a sufficiently long time, from a defence force well supplied with wireless telegraphy, fast scouts, and aerial observation, would now be a practical impossibility. But even if we suppose such an expedition to be able (under cover of fog, or by a complete surprise) to cross the North Sea unobserved, there remains the further difficulty of the landing. A place must be found where the invaders could obtain immediate control of supplies and communications; there are but half a dozen such places at most upon our eastern coast-line, and these are all prepared for a strenuous defence by land. If we add to the land defence a mine-field and the presence of an unknown number of submarines, the attempt becomes one involving the certainty of immense losses, and the extreme probability of failure. Even the German war-lords have not yet made up their minds to the risk of seeing eight or ten divisions drowned in an hour.