In urgency - but not without a prolonged period of preparation. Tippett was never exactly a spontaneous artist. Unlike his younger contemporary, Benjamin Britten, who poured out pieces from early childhood, so that there was never any doubt as to what he would be, Tippett seems to have taken a conscious decision to become a composer in his early teens, without much obvious flair or idea of how to go about it. Simply acquiring an adequate technique, let alone discovering a personal voice, was to take him most of his twenties. When he embarked on the oratorio, he had still only completed three works he would ultimately keep in his catalogue - though one of them was the already vibrant and masterly Concerto for Double String Orchestra.
What evidently complicated his progress was a continuing internal debate as to the kind of artist he ought to be. And not only internal - for shocked, liked so many of his contemporaries, by the social deprivation of the Depression, and fearful of the darkening situation in Europe, he threw much of his energy in the 1930s into left-wing socio-cultural activities, even for a time becoming a Trotskyist. Despite involvement in such socialist jamborees as the first 'International Workers' Music Olympiad' in Strasbourg in 1935, he never seems to have subscribed to the belief of such agitprop composers as Hanns Eisler in the efficacy of music as direct action. But, at the very least, it was surely the duty of the artist to bear witness? For Tippett, the moment to put this to the test came in November 1938.
That month in Paris, a young Jew called Herschel Grynspan, fearful for the fate of his family, walked into the German embassy and shot the third secretary. Two days later the Nazis launched their infamous Kristallnacht pogrom. To have attempted to evoke anything so terrible with an expressionistic directness could at best only have demonstrated music's impotence in the face of such reality; at worst, it would have been an act of vilest opportunism. And here Tippett's very lack of spontaneity - his need, if sometimes laboriously, to study, consult and think things through - may well have been his salvation.
If anything positive, no matter how tentative - anything conducive to the collective good - were to be made of the horror, he seems to have concluded, it could only be through the containing of the material within a formalistic scheme which would serve at the same time to inhibit any egotistic indulgence by its composer.
The traditional scheme for such collective material was, of course, Baroque oratorio. And Tippett had long been fascinated by the tripartite design of Handel's Messiah, with its prophetic first part, its central narrative of Christ, and its final drawing of spiritual conclusions. His own 'Modern Oratorio', as he provisionally called it, would therefore open with an ominous evocation of the nations on the brink of war and substitute for Christ the figure of Grynspan, objectified as the universal scapegoat.
For a spiritual resolution in a world without god, admittedly, the best he could come up with was a Jungian injunction to acknowledge and reconcile the light and dark sides of the psyche. But taken together with the work's underlying movement from winter to spring, this could at least provide some sense of renewal - and the music would do the rest. All he needed now to draw his audience in was a contemporary equivalent to the function that chorales had served in the Passion music of Bach, and soon he felt he had found it in the form of five, strategically placed, spirituals.
At this point, Tippett asked T S Eliot, his informal mentor, if he would write the text. Eliot, appraising the simple lines and quaintly biblical images of the summary draft, declared that Tippett had more or less done the job himself. The actual composition of the 65-minute score took some 18 months, often under the most fraught conditions. During the raid that prompted the 1940 letter, Tippett had actually been drafting the central number of the whole work: a precipitate, minute-long chorus that evokes the pogrom all the more implacably for being cast in strict invertible counterpoint. Around the same time he was appointed director of music at Morley College in Lambeth - a development that served to focus all his political impulses back on to the running of a generous and innovatory musical community, only briefly interrupted by his three-month prison sentence in 1943 as a staunch pacifist.
On 19 March 1944, and partly at the instigation of Britten, A Child of Our Time at last received its first performance, in the Adelphi Theatre, by a distinguished line- up including Joan Cross and Peter Pears, a choral contingent from Morley and the London Philharmonic under Walter Goehr. By now, its original subject matter had been overtaken by five years of even more appalling events, but it made a deep impression - and, reportedly, still deeper in Continental performances soon after the war. Meanwhile, following a few further Morley years, Tippett was to withdraw into a more personal visionary world. In the 1950s, this was to yield a sequence of surpassingly lyrical masterpieces. Later he would seek to re-engage in more angst-ridden issues of the times. But never again would he involve himself politically or musically so closely in the concerns of a community.
This Tuesday on Radio 3, Simon Rattle conducts a 50th anniversary performance. Those who approach the work for the first time from the standpoint of Tippett's later work, let alone from what these days glossily passes for conviction music, may find A Child of Our Time disconcerting. The words are plain, sometimes nave; the structure is terse to the point of short- windedness; the musical idiom less than fully personal or integrated - ranging from a post-Parry English choralism, by way of popular allusions, to an uneasy 'modernist' chromaticism and proving particularly tricky to bring off in those bleakly innovatory recitatives. Despite patches of contained passion and touches of Tippett's inimitable rhythmic verve, the overall effect is of a sobriety that can (dare it be said?) make even Britten's doubtlessly 'greater' War Requiem sound a mite theatrical - that other, belated English pacifist protest which, in pitching its action in the First World War, also arguably evaded the more complex moral challenge of the Second.
Yet the austerity, the fallibility even, of Tippett's oratorio are precisely what defy its sensationalisation. To hear him doggedly putting word together with word, and note against note, towards a hopeful communicability - and then, at the end, abandoning his home-made text for a hard-earned surge of purely musical exultation - is to be reminded not only of the modest function art can decently serve in extreme times, but of a more altruistic musical world - before considerations of a composer's image demanded that he should project 'attitude', and compassion became a mere commodity in the sales campaigns of record companies.
Simon Rattle conducts 'A Child of Our Time', Tuesday 7.30pm BBC Radio 3