Some 8,000 women worked at Bletchley Park, about three quarters of the work force. Before the attack on Pearl Harbor, the US Navy sent letters to top women's colleges seeking introductions to their best seniors; the Army soon followed suit. By the end of the war, some 7000 workers in the Army Signal Intelligence service, out of a total 10,500, were female. By contrast, the Germans and Japanese had strong ideological objections to women engaging in war work. The Nazis even created a Cross of Honour of the German Mother to encourage women to stay at home and have babies. Effect on the war The exact influence of Ultra on the course of the war is debated; an oft-repeated assessment is that decryption of German ciphers advanced the end of the European war by no less than two years. Hinsley, who first made this claim, is typically cited as an authority for the two-year estimate. Winterbotham's quoting of Eisenhower's "decisive" verdict is part of a letter sent by Eisenhower to Menzies after the conclusion of the European war and later found among his papers at the Eisenhower Presidential Library. It allows a contemporary, documentary view of a leader on Ultra's importance: There is wide disagreement about the importance of codebreaking in winning the crucial Battle of the Atlantic. To cite just one example, the historian Max Hastings states that "In 1941 alone, Ultra saved between 1.5 and two million tons of Allied ships from destruction." This would represent a 40 percent to 53 percent reduction, though it is not clear how this extrapolation was made. Another view is from a history based on the German naval archives written after the war for the British Admiralty by a former U-boat commander and son-in-law of his commander, Grand Admiral Karl Dönitz. His book reports that several times during the war they undertook detailed investigations to see whether their operations were being compromised by broken Enigma ciphers. These investigations were spurred because the Germans had broken the British naval code and found the information useful. Their investigations were negative, and the conclusion was that their defeat "was due firstly to outstanding developments in enemy radar..." The great advance was centimetric radar, developed in a joint British-American venture, which became operational in the spring of 1943. Earlier radar was unable to distinguish U-boat conning towers from the surface of the sea, so it could