Planned Special Forces attack on New York, 1943
Probably the most ambitious midget submarine operation during the entire Second World War would have been the Italian attack on New York planned in 1943. Italy’s elite frogman unit, Decima MAS, planned to transport a CA Class midget submarine to New York harbor on a larger sea-going host submarine, and then use the midget submarine to deliver 'Gamma' frogmen to attack shipping with limpet mines. The mission required the perfection of entirely new ways of operating Special forces from submarines, and could have inflicted a significant bloody nose on the allies seemingly impenetrable US base. The mission was only thwarted by timing and circumstances when the host submarine, Leonardo da Vinci was sunk by depth charges in May 1943 before the mission was launched.
Italy entered World war Two on the Axis side and launched a number of naval special forces missions against British harbors in the Mediterranean which were within easy reach from Italy. The US Mainland was generally thought out of range however, so a strike to New York would have a strong phycological impact.
Having invented the modern concept of using Special Forces for ship attack with the Mignatta in 1918, the Italian Navy continued to lead the world in terms of undersea special warfare into World War Two. The main means of attack were MT explosive boats (which ran on the surface but could be launched from submarines) and SLC ‘Maiale’ two-man human torpedoes. These craft, particularly the Maiale, proved effective in combat, but they were not considered practical for the ambitious attack on New York.
MT explosive boat and SLC Maiale
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Instead Decima MAS converted a small CA-Class midget submarine to be launched from a host submarine. The all-electric CA-Class had been built in the 1930s for harbor defense and were normally armed with two torpedoes:
Original artwork - CLICK for HIGH-RESOLUTION image. 
For the New York attack the CA-Class was heavily modified with the torpedoes removed and four large mines instead added to a remodeled superstructure. A diver lock-out compartment was built into the hull with hatches on both top and bottom. This allowed the frogmen to access the midget submarine directly from the dry interior of the host submarine.
Original artwork - CLICK for HIGH-RESOLUTION image. 
The plan was to deliver a CA midget submarine to the mouth of the Hudson River using a submarine of the Italian Atlantic Fleet which operated out of occupied France. It would then drive up the river, releasing frogmen who would place limpet mines on unsuspecting merchant ships there before rejoining the mini-sub and returning to the mother submarine.
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While the damage done to the War effort would have been minimal, the morale and strategic value would have been massive. The submarine Leonardo da Vinci was duly modified and trials carried out with the CA midget submarines being transported by rail from Italy. The arrangement involved replacing the forward gun with a cradle which positioned the midget submarine directly over the gun access hatch, thus allowing access between the two, a major factor in maintaining readiness during the voyage. The cradle was surrounded by a protective screen which gave rise to the nicknames Canguro (kangaroo) for the mother submarine and Cangurino (joey) for the CA.
The trials were deemed a success, but the old CA boats needed replacement with upgraded new-build examples so there was a delay in carrying out the plan, during which time the Leonardo da Vinci was returned to regular submarine operations. There was a further setback when she became the victim of depth charges in May 1943, effectively sinking the plan at the same time; it was over and never put into action.
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