I think it can be said that countries have at times won or lost wars based on efforts by their intelligence agencies. I remember reading one of Lex’s posts on the subject of the US nuclear submarine Scorpion. I doubt that we will never know what caused her sinking.
Reading up on John Walker awhile back, I shudder to think had we been at war with the Soviet Union, what would have been the outcome with our naval forces. He had given them the same kind information that we had on the Japanese in WW2 – cryptographic codes that let them know of not only naval unit movement, but intentions such as orders to launch missiles from our submarines.
I wrote on the Marburg Files, and how the efforts of Edward VIII may have helped the Germans in their invasion of France. One of the most amazing successful intelligence efforts was convincing the Germans that the D-Day invasion would occur at Calais, and not Normandy. It was so successful that even when Normandy was underway, Hitler believed that it was a feint and held his armor back waiting for the Calais invasion. Stalin refused to believe his spy telling him that the Germans had planned on invading the Soviet Union.
Aldrich Ames decided to betray his country – knowingly causing the deaths of 13 CIA assets in the Soviet Union, because he was going though a costly divorce and had a Columbian girlfriend with the tastes of Imelda Marcos.
Money was also the motivation of Robert Hanssen, who informed the Soviets of our counterintelligence efforts of the FBI. He cause the deaths of a number of Russians who were working with us. He was finally caught by the smallest of oversights.
I just finished a fascinating miniseries on Spycraft, which dealt with techniques, means of communication, and motivations of spies. It includes interviews with former CIA officers, and a colleague of Ames. It’s on Netflix.
I will also have to reread a book I read years ago, Secrets of D-Day. There were some surprising revelations. One of the biggest mysteries to me was the loyalties of the head of the German Abwehr, Wilhelm Canaris. How much he helped the Allies will probably never be known. He was executed by the Nazis in the closing days of the war.