Quoting Frank Joseph’s The Axis Air Forces: Flying in Support of the German Luftwaffe, pages 7–9:

Perhaps most surprising of all, several USAAF personnel, mostly prisoners-of-war, volunteered to fight for [the Axis], but 2nd Lieutenant Martin James Monti was the only American to actually defect with an aircraft. Hundreds of other Allied aircrews went AWOL during the European conflict by fleeing to neutral countries, such as Portugal, Turkey, or Sweden.

In Switzerland alone, 186 Liberator and Flying Fortress heavy-bombers, together with additional numbers of other USAAF warplanes, sought refuge. By contrast, just a single Italian fighter pilot fled to Switzerland.⁴

But on October 29, 1943—the 21st anniversary of Mussolini’s ascent to power—Monti flew a Lockheed Lightning from the 354th Air Service Squadron, stationed at Pomigliano Airfield, north of Naples, to Milan, capital of the Duce’s new Fascist state, the [so-called] Italian Social Republic. He tended both his warplane and personal services to the “Axis war against Communist Russia.”⁵

But [Axis] authorities took more interest in the F-5A, an up-graded reconnaissance version of the P-38G, in which Monti arrived, sending it to Germany’s Rechlin test center for evaluation.

When his application to join the Luftwaffe was turned down, he enlisted in the SS Standarte Kurt Eggers, a propaganda arm of the Waffen-SS in Berlin, rising to the rank of Untersturmführer, as a propagandist. A fellow American in the same unit was Louisiana-born Peter Delaney, an SS-Haupsturmführer, who later enlisted in the Légion des Volontaires Français—composed of SS volunteers from France—because he spoke fluent French.

Another U.S. comrade was New Yorker Roy Rickmers, awarded the Knight’s Cross on March 26, 1943, for outstanding heroism while serving with the 320. Infanterie-Division, which had been cut off at Liman, southeast of Kharkov, by a Soviet advance; Rickmers was the only American to receive this high Wehrmacht award.

German documents show that five U.S. citizens were enlisted in the Waffen-SS by May 1940, and at least eight more fell in action by war’s end, although the total number of American volunteers has never been ascertained.

During April 1945, Martin Monti was still wearing his SS uniform when arrested in northern Italy by Communist partisans. They turned him over to American military authorities, who thereafter sentenced their former 2nd Lieutenant to 15 years imprisonment for desertion but granted him a pardon several months later on the condition that he join the U.S. Army. Following promotion to sergeant, he was arrested again, condemned this time to 25 years incarceration on charges of treason but paroled in 1960. He died 40 later in his Missouri home.

Another USAAF officer—an unidentified Major and former POW—was known to have participated in the Deutsche Volkssturm Wehrmacht (the German People’s National Militia) during the final defense of Berlin, where he was reported missing in action shortly before the capital fell. Dr. Josef Goebbels was diligent in destroying all records of individuals who volunteered from the United States or England to protect them from personal postwar consequences.

According to the British Fascist John Amery, who broadcast for the Third Reich throughout the war, “Three Royal Air Force airplanes have come over to us so far with their arms and equipment.”⁶ But none of the English POWs—some from the RAF—who joined the [Axis], served in the Luftwaffe; all enlisted in the British Free Corps of the Waffen-SS to fight invading Soviets near the west bank of the Oder River, in January 1945.

(Some emphasis added.)

Further reading: Renegades: Hitler’s Englishmen