of the Master Spy That Helped Win World War II (1976, 2000)
British Prime Minister Winston Churchill (who years ago had helped organize the "Secret War" of intelligence against the Nazis) requested that William Donovan be sent to the Balkans on Britain's behalf (Donovan was FDR's right-hand man in terms of intelligence, just as WS was Churchill's right-hand man). Churchill wanted Donovan to do what he could to frustrate Hitler's timetable for unleashing Barbarossa. Donovan left under an assumed name, but he intentionally kept his initials on his luggage (WJD) in order to draw attention to himself so others could operate in the dark.
German intelligence had Donovan tabbed as FDR's emissary since 1937, and they were aware that Donovan was in London . . . and in following Donovan, Hitler was led into the trap. While Donovan scattered clues wherever he could, WS left nothing on his trail; in essence Donovan's role was to be the "clown" providing cover for WS. In response to Charles Lindbergh
and the American First movement (the "Firsters"), FDR wanted to demonstrate that the true first line of defense for the US was in Europe, and even in the Mediterranean. WS let it be known in not-so-carefully coded radio transmissions that Donovan was the source in the US for what Britain desperately needed during the war (e.g. bombsights), which was enough to explain Donovan's movements as an American "neutral".
Ultra had known for quite some time about Hitler's plan to invade the USSR, but the British couldn't reveal how they discovered that information, which made it impossible to change Stalin's mind. On 18 December 1940 Ultra intercepted/decoded a German transmission that confirmed Barbarossa, but Britain could do little with that intelligence. Hitler was about to unleash the greatest military surprise in history, and Stalin was an unwitting accomplice. So, Churchill thought, if Stalin refused to see the reality of the situation, then he needed to figure out a way to delay the Wehrmacht so that the Russian Winter would hit them hard and buy the USSR some time . . . it was under those limitations that Donovan headed to the Balkans to stir up trouble.
Donovan also "allowed" German agents to take "top secret" documents from his room that stated that the US would intervene in the Balkans if Hitler went too far in the region. Donovan went so far to pretend he was totally drunk at a party (he was a teetotaler), despondent that he had lost the documents . . . and Joseph Goebbels, the Nazi Minister of Propaganda, bought the ruse. The forged documents the Germans had taken from Donovan were incredibly well-made by a small unit WS had set up near Toronto (Camp X), and the Germans believed that the US was cooperating with the British Secret Service, who was viewed with both respect and hatred by the Nazis.
Donovan made it clear to Tito that if Prince Paul (the de facto leader of Yugoslavia) caved to the Nazis, he needed to revolt; when Donovan left Belgrade, the Germans were nervous and trigger-happy. On 22 March 1941 a Nazi ultimatum was presented to the Yugoslav government, and officers within the military overthrew Prince Paul, and Tito waited on the sidelines, biding his time. On 25 March 1941, Donovan, who had returned to the US, made a national radio address in which he spoke of the courage of those in the Mediterranean that were standing up to Hitler. The speech was designed to provoke Hitler into another fit of rage, which it did; it was known to Churchill, et al by then that Hitler made mistakes when he was in true fit that wasn't staged for effect.
Donovan's speech was recorded/filmed and was widely distributed, and it received a tremendous amount of global publicity. As a result, FDR could circumvent the Neutrality Acts by labeling the Mediterranean Sea, the Red Sea, and the Persian Gulf as as no longer combat zones.
The resistance forces in Yugoslavia went active, and they were similar to those in Belgium against Germany in World War I in that once again the Germans were delayed in achieving a greater goal, needing to use divisions that were supposed to be part of Barbarossa to deal with Yugoslavia, and then resistance in Greece and Turkey. Field Marshal Wilhelm Keitel complained that Barbarossa could be delayed by four weeks, but the reality would be closer to six weeks. Hitler's Operation Punishment in the Balkans would commit Germany to being involved in a quagmire against guerilla forces that would last the rest of the war. The key to initially committing the Germans to a quagmire was to overthrow the regime of Prince Paul in Yugoslavia, which was something Britain had wanted to occur for at least five months, but was unable to do . . . until Churchill enlisted the services of William Donovan, and the Nazis overreacted.
What saved the USSR was that Hitler was delayed enough by the British shenanigans in the Balkans that the Russian Winter came into play; the delay was something Stalin and most Russians to this day don't either know or appreciate.