In 1955 the Boston Symphony Orchestra celebrated its seventy-fifth anniversary and Martinů, who had been a friend of the orchestra's chief conductor Charles Munch since the 1930s in Paris, dedicated one of his best works to Munch and his orchestra. The Fantasies symphoniques, H 343 are often designated imprecisely as Martinů's Symphony No. 6, but whereas each of the five symphonies he composed between 1942 and 1946 during his American exile has a strict form, the Fantasies symphoniques constitute a clear turning point in his output: in his own words, they represent a "departure from symmetry in the direction of fantasy". This formal freedom and the constant thematic metamorphoses we find here are the principal traits of Martinů's late style. The orchestration of the Fantasies symphoniques is fascinating especially in the amazing density of its sound spectrum: commentators often speak of neo-impressionism in this regard. In some places that are especially important structurally - for example in the wind quintet in the third movement - the otherwise dominant complexity gives way to absolute simplicity and transparency. In regard to the formal aspect of the work, Martinů wrote that compared with his early compositions constructed in strict form the Fantasies symphoniques are de facto "without form". Nevertheless "something holds them together – I don't know what, but it has a single line and I've expressed something." He didn't say a word about the equally-important fact that this work was also a sort of private confession for him. This is what lends it – as also in the case of the Double Concerto for Two String Orchestras, Piano, and Timpani, H 271, the Third Symphony, H 299, and the Fifth String Quartet, H 268 for example – a sense of considerable urgency.
In the title of the work (which originally was to be Nouvelle symphonie fantastique) one can see a clear reference to the Symphonie fantastique by Hector Berlioz - a composer whom Martinů greatly respected and whose orchestral scores he had in his library. The Fantasies symphoniques were written during a tense period of the Cold War, which they indisputably reflect. An enthusiastic Boston critic rightly observed after the premiere that there is something frightful in this music, something that holds the listener in breathless suspense for the duration. The depressing political situation was accompanied in the musical sphere by the aggressive rise of serial composers and an associated long-term defection on the part of many music critics from the renowned composers of the pre-war modernist movement. Clearly, these circumstances may be one of the reasons why Martinů took more time to complete the work than was usual for him: he began composing it in 1951 in New York and didn't finish it until 1953 in Paris.
The composer's long-time friend and biographer Miloš Šafránek found the Fantasies symphoniques to be a very personal, non-derivative, and sincere work with powerful and dramatic music, sometimes nostalgic, tragic to the point of violence but firm in its faith. Martinů himself wrote about this work much more laconically in the program for the premiere in Boston, evidently partly in the effort to suppress the biographical impulses that were important for its origin: "There is just one reason for this work, and that reason is clear and certain for me. I wanted to write something for Charles Munch. He has a powerful effect on me and I like his spontaneous approach to music, in which the music takes on its form freely and flows without restraint, following its own motion. An almost undetectable slowing or hastening suddenly gives life to the melody. Thus it occurred to me to compose a symphony for him, which I wanted to title "Fantastic". [...] One of my little fantasies is that I used a quotation of several measures from another work of mine - the opera Julietta - which in my opinion is perfectly apt here. It has the nature of a fantasy. I did this for myself in a sense, because I like the special color of the orchestra in it, and, thinking I would never again hear this opera of mine, I wanted to hear again at least these several tones which I wrote from memory."
The fact that Martin allegedly intended to use three pianos in the orchestra can certainly be regarded as another of his fantasies. In view of the difficulties this would pose in performance he gave up the idea in the end and, in contrast to all five of his previous symphonies (and many other orchestral works) he didn't use even one. The glorious premiere of the Fantasies symphoniques took place in January 1955 in Boston. The Boston Symphony Orchestra was conducted by Charles Munch, who then presented the work in other North American cities. The Music Critics Circle of New York City honored the Fantasies symphoniques as the best orchestral work performed for the first time in New York in 1955.
Aleš Březina, programme of the Bohuslav Martinů Festival's concert, December 19–20, 2002