G.I. G-Men: The Untold Story of the FBI’s Search for American Traitors, Collaborators, and Spies in World War II Europe

by Stephen Harding

The explosive, untold true story of the band of patriotic FBI agents who went undercover to hunt down US traitors on foreign soil during World War II, and later investigated war criminals in the lead-up to the famous Nuremburg trials--a riveting real-life spy thriller from the bestselling author of The Last Battle . . .

They collaborated with Nazis and Fascists. Conspired against Allies in World War II. Committed unthinkable acts of treason. And triggered a secret manhunt as harrowing as any Hollywood cliff-hanger. The mission was part of the FBI's "European Operation." The targets were US citizens plotting against their own country. The goal: to identify and capture these traitors hiding in the shadows of war-torn Europe. To accomplish this, a small group of federal agents assumed new identities to infiltrate underground networks, interrogate key suspects, and expose the enemies within the Allied ranks. It is one of the most fascinating spy stories of World War II--and one that's never been fully told. Until now. . . .

Using newly declassified documents obtained through the Freedom of Information Act--as well as exclusive interviews with family members of the FBI agents and unpublished accounts of American citizens accused of treason--this meticulously researched book provides shocking new details behind this crucial WWII operation. From J. Edgar Hoover's attempt to expand FBI operations overseas to the agency's covert ties to Britain's MI5; from the shocking exposure of espionage activities in France and Italy to the final convictions of Nazi war criminals at the Nuremberg trials--this incredible saga of spy-counterspy is brought vividly to life by author Stephen Harding in what is destined to be classic of World War II literature.

G.I. G-Men is a must-read for military and history buffs, espionage fans, and anyone who loves a great adventure story.

Genres: History, Nonfiction, Crime

416 pages, Hardcover

First published February 24, 2026

About the author

As a defense journalist Stephen Harding covered the conflicts in Northern Ireland, the Middle East and, most recently, Iraq. The author of seven books and some 300 magazine articles, he specializes in military, aviation and maritime topics."

Reviews

[Darya Silman · Rating 4 out of 5]

While not a definitive history of the espionage/counterintelligence activity of the period, G.I. G-MEN: THE UNTOLD STORY OF THE FBI’S SEARCH FOR AMERICAN TRAITORS, COLLABORATORS, AND SPIES IN WORLD WAR II EUROPE by Stephen Harding is a compellingly readable work that provides insights for history buffs and amateurs.

The word ‘traitor’ usually awakens a feeling of betrayed trust and broken loyalty to one’s country. In people’s imagination, treason usually requires something of grand proportions. Yet,the very definition of treason in America was deliberately limited to prevent the government from abusing the term. Article III, Section 3 of the U.S. Constitution states,

Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court.

With that in mind, FBI agents, under the guidance of the FBI’s famous director J. Edgar Hoover, infiltrated war-torn Europe in 1944. They aimed to detain American-passport holders who, nevertheless, broadcast Nazi propaganda.

Chapters of G.I. G-MEN follow a geographic structure rather than combining traitors of similar backgrounds. There are many presumed traitors, some of them famous (like Ezra Pound), others small figures, so without keeping notes, it’s hard to remember every one of them. Most of them escaped legal prosecution because of the narrow definition of the law. As with another Stephen Harding’s book I read, it seems that the author wants to put every detail, significant or otherwise, into the book. The narrative moves briskly from one topic to another, blending people’s lives and history into an intriguing case study. And ironically, the story that most stuck with me isn’t about the traitors but a stolen decypher machine: millions of dollars wasted because of some drunk soldiers (never caught, by the way).

All in all, G.I. G-MEN entertains and educates at the same time. So I can recommend it to anybody interested in the FBI and WW2. For me, it was a complementary read after finishing a comprehensive J. Edgar Hoover biography last summer. However, this book would be interesting even to those who have no previous knowledge of the subject.

I obtained an advanced review copy through Netgalley.

[Dan · Rating 4 out of 5]

My thanks to NetGalley and Kensington Publishing- Citadel for an advance copy of this book that looks at the efforts of many to try and identify, bring to justice and punish those Americans and other sympathizers who worked for the Axis powers in Europe during the Second World War.

When trying to be successful in something, many of us are pretty myopic about what comes after. Success is the key, all else is well just noise as the saying goes. Well the noise can sometimes come back and bite you, or can just keep being noise, and that could be a bad thing. Military planners are quick to plot out how to win the war. The peace, well that is messy. As is justice. What happens to sympathizers, the fellow travellers who helped the enemy when they could. Shouldn't they be brought to justice, if not even just exposed to the light. Peace though is not just messy, peace is the time between the next battles, as many at the end of World War Two were already looking at, and some were looking forward too. One man, one paranoid man, with a love of control, and an even bigger love of using his organization to gain himself power, had been scheming to handle the peace even before the United States went to war. All shown in this fascinating book, a time that seemed to be missing from history. G.I. G-Men: The Untold Story of the FBI’s Search for American Traitors, Collaborators, and Spies in World War II Europe by journalist and historian Stephen Harding is a history of the FBI and its battles against spies and collaborators, from the start of the war, until well into the Cold War.

The book begins with two men arriving in Europe, dressed as officers, but with a different sidearm than most in the military. A sidearm used by FBI agents in the United States. These men were the vanguard of a force, developed by J. Edgar Hoover, head of the Federal Bureau of Investigations, to find American traitors, spies, and anyone who could threaten the peace. The book goes back into the early days, the so-called Phoney War between the Axis and Allies, when Winston Churchill was just made Prime Minister. When asked how he planned to fight the war, Churchill said, involve the Americans. From there we travel to America where the country had no interest in European conflicts, and in many cases, like today, had a love of Fascism. German agents were keeping track of boats carrying supplies, providing information to German High Command for U-Boats to sink. Harding explores the shared interests that both Hoover and British intelligence had, with both sides providing training and information. Harding looks at the early efforts to spy on foreign countries in South America with pro-German feelings. Leading to the formation of FBI agents to be set loose, tracing spies, Americans, and rat lines that German high value targets were using to escape justice.

A very different look at the post-War world, one that I don't remember reading much about. I knew about the Nuremberg trials, but knew little about what happened to Americans who were fellow travellers to the Nazis. Harding has done an excellent job in researching and finding stories, stories that many people would rather forget. The interviews with family members of the agents was very revealing, and really put a face to the people who were trying to do the right thing. Harding is a very good writer, keeping the narrative moving well, not being overwhelming with names, and even more crafting a very thrilling narrative. I enjoyed this book quite a bit, and even more learned quite a lot.

A book that fans of true crime, espionage and World War II will enjoy quite a bit. Again there is a lot of information, presented well, and written in a very enjoyable style. This is the the first book I have read by Stephen Harding, I look forward to reading a lot more.

[Page Inman · Rating 4 out of 5]

If you want an in-depth look at the history of the FBI’s involvement in World War 2, then this book will let you peek behind the curtain and see how spies and traitors were discovered and how they were punished and how it affected the war efforts.
A must read for history buffs and fans of espionage.

[Rachel · Rating 1 out of 5]

I won this book in a goodreads giveaway so my opinions are my own. this book is very dull. I'm very interested in WWII history but this one didn't grab me

[Abigail Jones · Rating 4 out of 5]

very intresting

[Gabriel Thomas Malanchuk · Rating 5 out of 5]

My wife won this book through the Goodreads giveaway. Very interesting book! The work the FBI did to ferret out traitors to our country during World War Two is amazing. I highly recommend this book.