The limits of legitimate enterprise across seas of which the command although firmly gripped is not unassailably established, are perhaps best illustrated by the story of Craig's expedition to Malta and Sicily towards the close of the Trafalgar campaign. This remarkable episode, which has received less attention than it deserves from most historians, has been represented by Mr Julian Corbett in his instructive work on _The Campaign of Trafalgar_ as the masterly offensive stroke by which Pitt hoped to abate, and, if it might be, to overthrow the military ascendency which Napoleon had established in Europe. That view has not been universally accepted by Mr Corbett's critics, but the episode is entitled to close attention for the light it throws on the central problem of naval warfare. Pitt had concluded a treaty with Russia, which involved not merely naval but military co-operation with that Power in the Mediterranean. Craig's expedition was the shape which the military co-operation was to take. It consisted of some five thousand troops, and when it embarked in April 1805 it was convoyed by only two ships of the line in its transit over seas which, for all the Government which dispatched it knew, might be infested at the time by more than one fleet of the enemy. Here, then, is a case in which the doctrine of the command of the sea and the principle of the fleet in being might seem to be violated in a crucial fashion. But the men who directed the arms of England in those days knew what they were about. Long before they allowed the expedition to start they had established a close and, as they thought, an effective blockade of all the Atlantic and Mediterranean ports in which either French or Spanish warships ready for sea were to be found. Nevertheless we have here a signal illustration of the essential difference between a command of the sea which has been made absolute by the destruction of the enemy's available naval forces--as was practically the case after Trafalgar--and one which is only virtual and potential, because, although the enemy's fleets have for the time been masked or sealed up in their ports, they may, should the fortune of war so determine, resume at any time the position and functions of a true fleet in being. On the strength of a command of the sea of this merely contingent and potential character Pitt and his naval advisers had persuaded themselves that the way to the Mediterranean was open for the transit of troops. Craig's transports, accordingly, put to sea on April 19. But a week before Villeneuve with his fleet had left Toulon for the last time, had evaded Nelson's watch, and passing rapidly through the Straits, had called off Cadiz, and picking up such Spanish ships as were there had disappeared into space, no man knowing whither he had gone. He might have gone to the East Indies, he might have gone to the West Indies, as in fact he did, or he might be cruising unmolested in waters where he could hardly