I wonder if he had any idea they were Jewish...

In the book, "The Groucho Letters" (genuinely well worth the trip to the library), it was mentioned that Hitler was especially fond of the brothers' last name, stating in a letter how much he enjoyed their antics. "Duck Soup" was the brothers' reply -- which, since the ultimate source is the immortal Groucho, may or may not be true.

12 Sample pages.
Image Credit: Amazon.com.

Trivial Bits ‘n’ Pieces
* At 68 or 70 minutes (with the unedited To War!), the film is the second shortest of the Marx Brother’s films after Horse Feathers (1932) at 66 minutes.

* Duck Soup was the fifth largest grossing film in America of 1933.

* By this point in their careers, all of the Marx brothers were on such bad terms that they refused to travel together and often did not turn up to rehearsals when they were supposed to. All of the brothers were renowned for their drinking, their gambling and their womanizing. This also took it’s toll on the time taken to produce the film.

* An indication of this is the tiny part played by Zeppo (Bob, Firefly’s aide). Zeppo was becoming increasingly unhappy with having to play straight parts, and the studios were also keen to replace him with more famous male leads. This was the last time he appeared in a Marx Brothers film.

* Harpo had just visited Germany before the film was shot, and was already concerned by Hitler’s treatment of the Jews. Again is another consideration in the anti-war sentiments of the film.

* The film was banned in Italy by Mussolini.

* Francois Truffaut claimed this and Shoulder Arms were the only valid anti-war films of the era, because the atmosphere of each steadfastly refused to take war seriously.

* The scene in which Chicolini, Pinky and the Peanut vendor argue is a close parallel to the major plot; showing how neighbors with apparently no aggressive intentions can be drawn into conflict. As with the Freedonia/Slyvania war, the fight is entirely orchestrated by the Marx Brothers for no real reason.

* In every shot of the last scene, Groucho wears a different army uniform. None of these are First World War uniforms.

* Margaret Dumont had not been in the previous film, but was by now an established part of the ensemble cast as the “straight” foil to the brothers. Groucho always maintained that the reason she was so perfect in these roles was that she never really understood the films. She was a classically trained actress/opera singer, and her performances are always melodramatic and extremely static. The brother’s frequent ad-libbing confused her, and this can often be seen in the hesitation before she delivers her lines. She was also extremely uncomfortable with physical contact, hence the jokes in the films where Harpo offers her his leg. This lack of poise can be seen in Animal Crackers, where at one point she seems to completely lose her temper and shoves Harpo away from her. This was a moot point however, and possibly just another of Groucho’s endless lies. When Margaret Dumont and Groucho re-enacted a scene from Animal Crackers on the television program The Hollywood Palace in 1965, Dumont was unable to keep a straight face throughout the performance. In The Marx Brothers Encyclopedia (1996), Mitchell concludes that “It is quite possible that Margaret Dumont had succeeded in putting one over on Groucho, who genuinely believed her to be without humor” (p.88).
(Source)

Dan