so that he could observe the raid from the Air Ministry roof. Ultra intelligence considerably aided the British Army's Operation Compass victory over the much larger Italian army in Libya in December 1940 – February 1941. Ultra intelligence greatly aided the Royal Navy's victory over the Italian navy in the Battle of Cape Matapan in March 1941. Although the Allies lost the Battle of Crete in May 1941, the Ultra intelligence that a parachute landing was planned, and the exact day of the invasion, meant that heavy losses were inflicted on the Germans and that fewer British troops were captured. Ultra intelligence fully revealed the preparations for Operation Barbarossa, the German invasion of the USSR. Although this information was passed to the Soviet government, Stalin refused to believe it. The information did, however, help British planning, knowing that substantial German forces were to be deployed to the East. Ultra intelligence made a very significant contribution in the Battle of the Atlantic. Winston Churchill wrote "The only thing that ever really frightened me during the war was the U-boat peril." The decryption of Enigma signals to the U-boats was much more difficult than those of the Luftwaffe. It was not until June 1941 that Bletchley Park was able to read a significant amount of this traffic currently. Transatlantic convoys were then diverted away from the U-boat "wolfpacks", and the U-boat supply vessels were sunk. On 1 February 1942, Enigma U-boat traffic became unreadable because of the introduction of a different 4-rotor Enigma machine. This situation persisted until December 1942, although other German naval Enigma messages were still being deciphered, such as those of the U-boat training command at Kiel. From December 1942 to the end of the war, Ultra allowed Allied convoys to evade U-boat patrol lines, and guided Allied anti-submarine forces to the location of U-boats at sea. In the Western Desert Campaign, Ultra intelligence helped Wavell and Auchinleck to prevent Rommel's forces from reaching Cairo in the autumn of 1941. Ultra intelligence from Hagelin decrypts, and from Luftwaffe and German naval Enigma decrypts, helped sink about half of the ships supplying the Axis forces in North Africa. Ultra intelligence from Abwehr transmissions confirmed that Britain's Security Service (MI5) had captured all of the German agents in Britain, and that the Abwehr still believed in the many double agents which MI5 controlled under the Double Cross System. This enabled major