Where, however, contact is not certain, the invasion over an uncommanded sea may succeed by evasion of the defender’s battle-fleet, as it did in the case of Napoleon’s invasion of Egypt.  But that operation belongs to an entirely different category from that which we are now considering.  None of the factors on which the traditional system of British defence is based were present.  It was an operation over an open sea against a distant and undetermined objective that had no naval defence of its own, whereas in our own case the determining factors are permanent naval defence, an approximately determined objective, and a narrow sea where evasion by any force of invasion strength is impossible.  Napoleon’s exploit was in fact nothing more than the evasion of an open blockade which had no naval defence beyond it.  The vital importance of these things will appear as we proceed and note the characteristics which marked every attempt to invade England.  From such attempts we of course exclude the various descents upon Ireland, which, not being of invasion strength, fall into another class, to be dealt with hereafter.

Since the expedient of forcing an invasion by the strength of a powerful battleship escort has always been rejected as an inadmissible operation, the invader has had no choice but to adopt a separate line for his army, and operate with his fleet in such a way as may promise to prevent the enemy controlling that line.  That, in short, is the problem of invasion over an uncommanded sea.  In spite of an unbroken record of failure scored at times with naval disaster, continental strategists from Parma to Napoleon have clung obstinately to the belief that there is a solution short of a complete fleet decision.  They have tried every conceivable expedient again and again.  They have tried it by simple surprise evasion and by evasion through diversion or dispersal of our naval defence.  They have tried it by seeking local control through a local naval success prepared by surprise, or by attempting to entice our fleet away from home waters to a sufficient extent to give them temporarily local superiority.  But the end has always been the same.  Try as they would, they were faced ultimately by one of two alternatives—­they must either defeat our covering battle-fleet in battle, or they must close their own battle-fleet on the transports, and so set up the very situation which it was their main design to avoid.

The truth is, that all attempts to invade England without command of the sea have moved in a vicious circle, from which no escape was ever found.  No matter how ingenious or complex the enemy’s design, a determined hold on their army as the primary naval objective has always set up a process of degradation which rendered the enterprise impracticable.  Its stages are distinct and recurrent, and may be expressed as it were diagrammatically as follows:—­