For many years information about this work derived from a very limited number of brief references. The first to be published is also the only direct reference by a witness who we can assume with some degree of certainty had actually heard any of the music. Gustav Schwarz was an estate manager in Ronow, Bohemia, and in a newspaper interview about his 'discovery' of Gustav Mahler (Neues Wiener Journal, 4233 (6 August 1905), 10) recounted the following memories:

„Es war im Jahre 1874 oder 1875, als ich, ein eifriger Musikliebhaber, unter meinen Manuskripten auch nachgelassene Noten von Thalberg fand. Ein Freund, Herr Steiner, dem ich meinen Fund zeigte, sagte: „Ich weiß einen jungen Burschen, der das vom Blatt spielen kann.” Das reizte mich und ich lud den jungen Mann ein, zu mir zu kommen, er erschein, ein schmächtiger, unbeholfener kleiner Junge, der in Iglau das Gymnasium besuchte, Gustav Mahler. — Ich war von seinem Klavierspiel entzückt. Wenn er am Piano saß, wurde er ein ganz anderer Mensch und wie spielte er, mit welchem Gefühl, mit welchem Verständnis!

"It was in 1874 or 1875, when I, an enthusiastic music lover, found unpublished music by Thalberg among my manuscripts. A friend, Mr Steiner, to whom I showed my discovery, said "I know a young lad who can sight read." That exited me and I invited the young man to visit me; he appeared, a frail, awkward, slight youth, who was attending the Gymnasium in Iglau: Gustav Mahler. — I was delighted with his piano playing. When he sat at the keyboard he became a completely different person and how he played, with what feeling with what understanding.

Ich sagte ihm sofort: „Lieber Herr Mahler! Sie müssen Musik studieren.” Der junge Mensch gestand mir nun, daß seine stille Neigung seit Langem sei, aber er habe den Widerwillen seines Vaters zu beseigen und er glaubte nicht, daß so ohneweiters zu erreichen sei.

I immediately said to him, 'Dear Mr Mahler, you must study music'. The young man then explained to me that that had been his secret wish for some time, but he had to overcome his father's reluctance and he didn't believe that could be easily achieved.

Mahler bleibt Gast im Hause des Herrn Schwarz und dieser faßt den Plan, den jungen Menschen nach Wien zu führen und ihn zu rationellem Studium zu bringen...

Damals hatte er schon eine Oper fertig „Herzog Ernst von Schwaben”, die Herr Schwarz kannte und als sehr interessant bezeichnete.”

Mahler remained as a guest in the home of Mr Schwarz, who formed a plan to take the young man to Vienna and to get him some good training....

At that time he had already finished an opera, Herzog Ernst von Schwaben, which Herr Schwarz knew and described as very interesting.

Schwarz seems to have played an important part in the family's eventual decision to allow Mahler to study in Vienna, and reported that he had taken Mahler to see the pianist Julius Epstein, a professor at the Vienna Conservatoire, for an assessment:¹

Herr Schwarz traf püktlich mit dem jungen Mahler zusammen, sie fuhren nach Wien und hier suchten sie sofort Professor Epstein auf.... „Ich erkannte sofort die eminente Begabung Mahlers,” sagte Herr Schwarz „als ich zu Professor Epstein kam, war er gar nicht entzückt, das Klavierspiel Mahlers imponierte ihm nicht. Erst als Mahler eigene Kompositionen zum besten gab, wurde Epstein warm, erklärte ein über das andermal, die Sachen seien direkt ‚wagnerisch’ und fragte mich, warum ich ihn denn nicht telegraphisch nach Wien berfufen habe.”

Herr Schwarz met young Mahler as agreed, and they made the journey to Vienna, where they immediately called on Professor Epstein... 'I recognized Mahler's outstanding gifts at once,' said Herr Schwarz; 'but when I came to Professor Epstein, I found him anything but delighted, for Mahler's piano playing did not impress him at all. It was only when Mahler played him some of his own compositions that Epstein showed any enthusiasm and said over and over again that there were in direct descent from Wagner, and asked me why I had not sent him a telegramme asking him to come to Vienna.'

This reference to the Wagnerian style of Mahler's music hints that some of what Epstein heard may have been from a dramatic work, perhaps Herzog Ernst von Schwaben, and there is further evidence that this work did indeed play a role in his study at the Conservatoire (see below).

One other contemporary of the composer seems to have known about the opera, but probably never heard any of it. In his memoir of the composer, Guido Adler regretted the loss of Mahler's early works, and included the opera in his list of the works destroyed by their creator in the period 1877–9 (GA, 75, 96–7; ERGA, 60²). Adler and Mahler both grew up in Iglau (Jihlava) in Moravia, but Adler had left for Vienna in 1864, and it was only in the late 1870s that he became a friend of the composer: Adler's reference makes it clear that he did not know the work, so presumably Mahler had told him about it. The composer had also recalled it earlier, in a letter written in the summer of 1879 to a childhood friend, Josef Steiner (1857–1913) (GMB2, 9; GMSL, 55):

Da ziehen die blassen Gestalten meines Lebens wie der Schatten längst vergangenen Glückes an mir vorüber, und in meinen Ohren erklingt das Lied der Sehnsucht wieder. – Und wir wandeln wieder auf bekannten Gefilden zusammen, und dort steht der Leiermann, und hält in seiner dürren Hand den hut hin. Und in den verstimmten Tönen hör' ich den Gruß Ernst's von Schwaben, und er kommt selbst hervor and breitet die Arme nach mir aus und wie ich hinsehe, ist's mein armer Bruder; Schleier senken sich herab, die Bilder und Töne werden blässer...

Then the palling shapes that people my life pass me like shadows of long-lost happiness, and in my ears again resounds the song of yearning. – And once gain we roam familiar pastures together, and yonder stands the hurdy-gurdy man, holding out his hat in his skinny hand. And in the out-of-tune sounds I recognised Ernst von Swabia's salutation, and he himself steeps forward, opening his arms to me, and when I look closer, it is my poor brother;³ veils come floating down, the images, the notes grow dim...

It was a footnote to the original 1924 edition that first identified the reference to 'Ernst von Schwaben' as relating to the opera, and also indicated that the author of the libretto was the recipient of the letter. Knud Martner, who prepared the English edition of this collection of letters, is of the opinion that in most cases the footnotes are by the editor, Alma Mahler, based on information supplied by the addressee. Since Steiner had died in 1913, it was probably his widow who supplied the letter and the information to Alma Mahler. That Frau Steiner knew about the opera is confirmed in a letter from the Steiner's daughter, Annie, to Donald Mitchell, written in 1961 (DM2, 55–6):

...In the years 1875 and 1876 Mahler and my father spent some of their holidays at a farm in Ronow in Bohemia, at the home of one of my father's aunts. During the holidays my father wrote, or finished, the libretto of an opera, Herzog Ernst von Schwaben, and worked with Mahler on the musical score At the end of this holiday – as he often told my mother – they packed all the papers they were working on in a box and stored them in an attic room. When they returned to Ronow for a holiday in 1876 and wanted to continue their work, the papers were gone. My father's aunt just shrugged, and said there was such a mess of papers about that she had simply burned them when she tidied up the attic. The two young men were rather upset, but apparently did not write down the libretto or opera again – although my mother told us that my father, about thirty years later, played fragments of Herzog Ernst to her from memory.⁴

So, the destruction of the completed portions of the score was not an act of self-criticism by the composer, which perhaps explains why he might have discussed it with Adler. On the other hand, when he referred briefly to the opera in a conversation with Natalie Bauer-Lechner in the summer of 1896 (NBL2, 69), Mahler apparently made no reference to the extraordinary circumstances that prevented its completion, perhaps because it was more strongly associated in his mind with what he perceived as a major flaw in his education:

[Ich komponierte] eine Klavier-Violin-Sonate, ein Nocturne für Cello; für das Klavier alles möglich, und endlich eine Oper, zu der ein Schulkollege den Text mit mir schreib. Auf grund dieses Bruchstückes (denn ich kam nie dazu, sie zu vollenden) wurde ich später am Wiener Konservatorium von Hellmesberger (diesem Schaf) Überspringen von Harmonielehre und Kontrapunkt zu meinem größten Schaden in die Kompositions-Klasse aufgenommen.⁵

[I composed] a Violin Sonata, a Nocturne for cello, for piano everything possible, and finally an opera, for which a school friend wrote the text with me. On the evidence of this fragment (for I never got around to completing it) I was later accepted by Hellmesberger (that dope) into the Composition class at the Vienna Conservatoire, skipping Harmony and Counterpoint, to my great disadvantage.⁵

For all its fragmentariness this was clearly a work that had carried substantial, and diverse psychological significance for the composer and Miss Steiner's account makes this more tangible by contextualising the reference to the work in Mahler's 1879 letter and revealing that for Mahler both his brother and his opera had been lost or destroyed.

Herzog Ernst von Schwaben (Unfinished Opera)