“It’s really wartime music – a great deal of it incubated when I used to go up night after night in the ambulance wagon at Ecoivres and we went up a steep hill and there was wonderful Corot-like landscape in the sunset. It’s not really lambkins frisking at all, as most people take for granted.”

Ralph Vaughan Williams was talking about one of his most controversial and misunderstood pieces, A Pastoral Symphony, his third, which he completed in 1922. It’s easy to see where the confusion comes from: here is that master of nostalgic evocation calling a piece “pastoral”, immediately asking audiences to hear it – you’d have thought – as the acme of all things quaintly, gently rustic, the sound of an imagined idyll of English landscape turned into sound.

So perhaps the symphony’s mixed reception is partly Vaughan Williams’s own fault: had he originally called it simply Symphony No 3, he wouldn’t have planted that pastoral seed in the minds of his listeners and his critics. Constant Lambert said that its four movements – nearly all of them slow, lyrical, and strange – have a “particular type of grey, reflective, English-landscape mood [that] outweighed the exigencies of symphonic form”.

But it’s not just Vaughan Williams’s testimony that should make us realise that the landscape of A Pastoral Symphony isn’t some Arcadian part of Surrey – if it is about landscape at all, it’s rather the blasted terrain of the fields of horror of the first world war. In fact, throughout this symphony there’s a disturbing doubleness, in which images and ideas that are usually thought to provide consolation instead suggest emotional instability and ambiguity. The pastoral title is, I think, almost ironic, since what Vaughan Williams is doing in this piece is turning the idea on its head, so that instead of being a source of comfort, this pastoral is instead a confrontation with loss, with lament, with death. And it’s also a genuinely adventurous attempt to write a kind of symphony that no-one had attempted so completely before, the secret of which lies in another interpretation of Lambert’s idea that the piece rethinks those “exigencies of symphonic form”.

More on that later, but first, let’s hear the Pastoral as critique of the pastoral. The most obvious wartime memorial in the piece is the trumpet cadenza in the second movement, a dream of a Last Post-like fanfare that drifts into the music’s consciousness. In the frame of a pastoral, this military reminiscence is already seemingly out of place, and in fact there’s a specific memory that Vaughan Williams is invoking. In that “Corot-like landscape” that he saw during the war: “A bugler used to practise and this sound became part of that evening landscape and is the genesis of the long trumpet cadenza in the second movement of the symphony”. You would have thought this obvious reference would have alerted the symphony’s early listeners to the real location of this music, in the wake of the First World War, but that’s only the clearest of many ways in which the idea of the pastoral is subverted.

More generally, there’s the continual elusiveness of the music, something you hear from the start of the first movement. Vaughan Williams’s harmonic idiom in this symphony is continually slipping from one tonal centre to another, from one “mode” to another (different divisions of the scale), so that, for all the music’s superficially quiescent surface, there’s an unsettling feeling to the way the symphony moves. There are melodies and motives you’ll certainly recognise and hold in your brain when you’re listening to the piece, and there’s a network of connections between the main ideas in the piece that stretches across all four movements. Yet moment by moment, Vaughan Williams makes the ground slide beneath your ears, so to speak: and it’s not just the harmony, it’s the music’s hauntingly subtle orchestration, too, in which instrumental timbres seem to melt into one another.

Even the third movement, which functions as a kind of scherzo in the symphony, manages to throw you off balance with its lopsided dance rhythms, and especially the weightless music of Mendelssohn-like gossamer that ends the movement, suspending you in the ether rather than placing down on the earth. But the final movement is the quintessence of the symphony, with yet more slow, subtly tortured music framed by two solos from a wordless soprano, marked “distant” in the score, and often performed offstage. As Daniel Grimley shows in a brilliant essay on A Pastoral Symphony, this solo line moves from “relative stability to complete harmonic ambiguity”. And yet, the effect in performance is singularly devastating. After the symphony has wrenched itself to its most insistent and loudest climax, the music’s return to this lamenting song for the soprano is all the more moving. It is the sound of absence somehow made present, music that echoes with the lives lost in those French fields, and it’s the distillation of a pastoral symphony that’s really an anti-pastoral.

Which finally bring us back to Constant Lambert: in situating his symphony in this mode of slow, reflective concentration, Vaughan Williams risked forfeiting this piece’s “symphonic” credentials. Yet in its “critique and reimagining of the pastoral”, as Grimley has it, Vaughan Williams not only vindicated his personal vision, and the pain of his wartime experiences, but achieved a new idea of the symphony, too.

Five key recordings

Andrew Manze conducts Vaughan Williams’s A Pastoral Symphony at the BBC Proms on Sunday 17 August with the BBC Scottish Symphony Orchestra.