D-Day is behind them, and the Allies have seized the momentum on the ground and in the air. The pivotal Battle of the Bulge still lies ahead but, as G.I. G-Men tells the tale, liberated France is already mostly freed of German occupation. The mopping up behind the front is vigorously underway, too. The FBI is on the scene in Paris and beyond, investigating turncoat American propagandists and collaborators who’ve stayed behind to spread Nazi lies over the airwaves.

Stephen Harding’s new book documents the FBI’s involvement in Europe in the final year of the war and its immediate aftermath. It’s an exhaustive account of an initiative that in practice (it must be said) amounts to a supporting role in the traitor hunt, backing up reemergent French authorities, the U.S. Army, and the recently formed Office of Strategic Services (OSS), the precursor of the CIA.

Except for internal agency and military communications, as well as a smattering of participants’ memoirs, the G.I. G-men saga is largely an untold story. It begins, during the 1930s, with the FBI’s brilliant success in rooting out stateside German spy rings and illegal Nazi-advocacy organizations at a time when U.S. sympathies are leaning decisively toward “America First” isolationism.

The upshot of this achievement: FBI Director J. Edgar Hoover attempts to grab a piece of the overseas-counterintelligence action through a personal appeal to President Roosevelt. In response, FDR charges the FBI with rooting out German operatives in Latin America, where Nazi agents are running rampant. The G-men seize on the task in our hemisphere, performing well both pre-war and after America joins the fight in Europe and the Pacific.

Once the tide turns overseas, the FBI might seem ideally suited to spearheading the search for spies, collaborators, and Nazi sympathizers among American hangers-on in occupied Europe. The author vividly describes how Hoover’s men address this mission, arriving first in liberated Italy, then in France, fitted out in U.S. Army uniforms with officers’ insignia (hence the “G.I.” moniker in the book’s title).

Early in his account, Harding offers the case of poet Ezra Pound, a stay-behind in Mussolini’s Italy and a sporadic radio propagandist for the Fascist cause. Pound comes across as befuddled and erudite by turns, at times absorbed in his translations of Chinese verse and at others presenting himself at interrogation sessions with addled formality, arm held high in the Nazi salute. He’s captured by regular Army troops and treated poorly until one of the first G.I. G-men arrives to take custody and send him on to the States. (You may know the rest: Pound is tried, then confined for years in St. Elizabeth’s mental hospital in Washington, DC.)

The Pound episode is the most compelling and richly detailed of the cases in the book, but it also illustrates the difficulty inherent in creating a consistently fascinating narrative around the activities of the G.I. G-men. There’s too little challenge, flash, and follow-through in their real-life assignments to inject narrative momentum into their crusades. Harding strives valiantly to recount their cases with panache, and he writes exceedingly well, but the driving investigative rhythm of the FBI guys’ exploits is only sporadically present despite his best efforts.

One might blame the relative scarcity of evidence surrounding various culprits’ guilt, but that’s something of an organizational artifact, a remnant of apparently fierce inter-agency competition. Hoover despised “Wild Bill” Donovan, the founder of the OSS, his most ardent rival for hegemony over these turncoat investigations, and the OSS often won out in clashes with the G-men for the chief role in pursuing leads. The State Department also cut in on several occasions, claiming responsibility for a given inquiry and freezing out the FBI.

What’s more, many of the apparent traitors pursued by the G-men seem to get off entirely, either for lack of concrete evidence — say, a recording or reliable multi-witness testimony — or, stunningly, because of help from friends in the American elite establishment who protect well-connected suspects. These latter coverups are the most disheartening elements reported in this expertly researched but ultimately flawed volume.

The result: We’re left hoping for more comprehensive investigations leading to convictions — a vital element for sustaining whodunit-like interest even in factual accounts like this. As scholarship, G.I. G-Men is first rate, but as popular historical nonfiction, it falls a bit short.

Bob Duffy reviews frequently for the Independent.