Certainly Ultra contributed a great deal to winning the Battle of The Atlantic along with other contributions from ASV and the overall ability of the Allies to apply their MR the width of the Atlantic leading to the defeat the U Boat menace. Without this,the build up of men and materials crossing the Atlantic for the invasion of Europe would have been delayed,thus interrupting plans for the invasion of Europe in the summer of 1944. For any counter intelligence to break the codes of their enemy,there has to be sufficient wireless feedstock to evaluate.For instance wireless traffic of the Gestapo and Bormann's grip on his Gaullieters was such a low volume that there was never a chance of the Allies being able to penetrate their communications. As said .the opening of Operation Bodensee came as a surprise to the Allies in December 1944 because Hitler forbade wireless traffic before the launch. From early in the war the Germans were never in a position to establish a spy network in Britain.Ultra intelligence was the basis for the function of John Masterman's Double Cross Committee.German spies entering Britain were quickly apprehended....their comings and goings were known to Masterman and as far as I can ascertain not one lived to tell the tale except those who were "converted" and collaborated with Masterman's deception. Ultra intelligence was available to Bernard Freyberg,the NZ commander in the Battle of Crete where the intention of the Germans were well known to him.He failed to take advantage of the intelligence and to share it with his senior commanders since the intercepts were ranked as Most Secret they were for his eyes only. Freyberg's leadership has been brought into question over his part in the Battle of Crete where he had superior numbers over the Germans and failed to exploit vital intelligence about the enemy's intentions. An interesting part of intelligence history which the public had no idea of until Masterman ignored official regulations and went public in the early 1970s with his work of the Double Cross deception. In addition to what sources have been mentioned,Hugh Sebag-Montefiore's "Enigma...The Battle For The Code" is a source worth reading as is "The Secret War" by Max Hastings.