To recapitulate, the convoys were liable to attack by submarines (of which in 1942 the enemy were always able to dispose at least 20 against them) throughout the length of their routes, which for geographical and other reasons allowed of little variation. In any case, air reconnaissance invariably gave the enemy early intelligence of their movements. The efforts of the U-boats were reinforced by M.T.B.s, based in Crete and Pantellaria, which on more than one occasion were used with considerable effect. The powerful Italian fleet, with its bases situated within a few hundred miles of the convoy routes, was a constant menace. Lack of sea-room and the danger of mines in the Sicilian Narrows made it imprudent for capital ships to accompany the convoys from Gibraltar beyond Skerki Bank, and for the last 250 miles of the passage they had to rely on relatively weak forces of cruisers and destroyers for protection. In the eastern Mediterranean there were no British capital ships during the critical first nine months of 1942. Though no Italian heavy ship ever actually got within sight of a convoy, on two occasions its intervention was sufficient to render abortive operations to replenish Malta from the east, and it was the existence of this fleet " in being " which put an end to such attempts after the failure of operation " Vigorous " in June 1942.