The world was at war, America precariously poised on the sidelines. But already a second secret war was well underway with the United States very much in the thick of it. While he fought on the home front to consolidate the FBI's intelligence gathering power, J. Edgar Hoover was conducting an all-out campaign to make his agency America's first foreign espionage service--a campaign that would lead to an uneasy alliance with British intelligence in a brilliantly successful operation to undermine Germany throughout the Second World War. While pieces of the story have been told before, only now, in this work by FBI historian and former agent Raymond Batvinis, does this crucial chapter in the history of World War II, and of the FBI, received its full due. Taking up the tale begun in his acclaimed Origins of FBI Counterintelligence, Batvinis mines a wealth of heretofore untapped resources to expose Hoover's remarkable connivances and accomplishments in concert--and occasionally contention--with the Allies in outsmarting German intelligence. Hoover's Secret War opens up a world of spy rings, secret and double agents, surveillance, codes and ciphers, wire taps, microdots, mail drops, invisible ink, radio transmissions, and deception and disinformation as it tracks the warring nations spreading their intelligence tentacles throughout Europe and North and South America. As it documents the rocky evolution of the FBI's relationship with Britain's vaunted M15 and M16, the book brings to light the feud between Hoover and William Stephenson, director of the British Secret Intelligence Service's U. S. operation, BSC. Batvinis reveals how the agency gained access to ULTRA intelligence, thanks to the British decryption of the ENIGMA code, along with the strenuous efforts to keep the Germans in the dark about it. He uncovers eye-opening details of the FBI's participation in the famed "Double-Cross System, which effectively "turned" German agents against the Fatherland, among them a flamboyant, larger-larger-than-life playboy, a world famous French flyer, and a lecherous Dutchman. Batvinis tells for the first time how the Bureau manipulated these agents, and how it transmitted deceptive information critical to the Normandy landings, the Allied invasion of the Marshall Islands, and the atomic bomb program, among other matters. Rich with secrets and surprises worthy of the finest spy fiction, this true story of espionage and counterintelligence gives us our first clear look at the secret second world war, and a significant moment in history--for the FBI, for America, and for the world.