THEY came for a concert. They witnessed a revolution. They could not have foreseen it, or have realised it even afterwards, those elegant guests who assembled at Prince Lobkowitz's Vienna palace on a bitter December night in 1804 for the premiere of a new symphony by the wild and woolly Ludwig van Beethoven. Of course, many in the nervously expectant throng must have muttered, the man was a peerless pianist; but as a composer he sadly lacked the sense of proportion of “papa” Joseph Haydn or the late lamented Wolfgang Amadeus Mozart. Still, perhaps the 34-year-old iconoclast was starting to mellow at last.

He was not. Wham, wham. With the two opening chords of his symphony number three in E flat major, the “Eroica”, Beethoven thrust aside the old classical order, with its precise rules, and opened the door to the glorious excess of the romantic era. If ever there was a moment that changed music, at least western music, then this was it; more than the first hearing of Wagner's “Tristan und Isolde” in 1865, more than the notorious premiere of Stravinsky's “Sacre du Printemps” in 1913. It was Beethoven, and above all the “Eroica”, that made the rest possible.

That may seem too tall a claim. After all, while the sound, fury and inordinate length of the “Eroica” were shockingly new, Beethoven, by and large, still stuck to established symphonic form. And Mozart had been no mere spinner of elegant notes, no practitioner of art for art's sake; he often cheerfully flouted classical convention. True, but with Beethoven as never before individual human aspirations, fears and passion are central to the music, threatening to overwhelm its structure, albeit never (well, hardly ever) doing so. As George Bernard Shaw put it, in his single-minded determination to express his own moods Beethoven “anticipated with revolutionary courage and frankness all the moods of the rising generations of the 19th century.” And, one might add, those of the dying 20th century too.

How did it happen, the “Eroica”, that bolt from the blue? Beethoven's first two symphonies have their oddities as well as beauties, but nothing that really hints at the monster coming next. Did Beethoven's increasing involvement with the stage, including his music for the ballet “Prometheus”, incline him more to building drama into “pure” music? Did Napoleon provide the stimulus, as numberless accounts have since claimed? Beethoven initially admired the Frenchman as (like himself) a democratic revolutionary, and on the title page of the “Eroica” wrote “Grand symphony entitled Bonaparte”. He tore the dedication out after Napoleon proclaimed himself emperor. But the connection can be carried too far. “No. No. Is-a not Napoleon,” screamed an exasperated Toscanini, rehearsing the first movement; “Is-a allegro con brio.” Does the increasing deafness that afflicted Beethoven from 1802 offer the key? We know he came close to suicide; he wrote later that “nothing but my art held my hand.” It was the moment, surely, to compose something very special. Is the hero of the “Eroica” not Napoleon but Beethoven himself? Both men, maybe? Or all who triumph over adversity?

Perhaps all these elements combined to produce the critical mass from which the symphony erupted. We cannot know. We can only hear the results.