Here we are eight years into a new century, high time to start looking back at the last century and asking what the hell that was about. Critic Alex Ross, in his best-selling book The Rest Is Noise, takes a long, hard squint at musical Modernism in its context, by way of what he calls "the 20th century heard through its music." If Ross doesn't come up with a lot of answers, he gives us a comprehensive survey and an outstanding read—and, in the process, suggests a new and commonsensical approach to a vertiginous subject.
The 20th century was the most healthy, comfy, democratic, generally advanced period in history, and also the most murderous and totalitarian, both largely thanks to science and technology at the service of ideologies. Revolutions in politics and technology were paralleled by revolutions in the arts—or, rather, an ebb and flow of revolution and retrenchment that made up the patchwork we still call Modernism.
In his book, Ross doesn't attempt a grand unified theory of Modernism, and that's probably wise. The century was a maze of crosscurrents. In 1912, Igor Stravinsky took music into a sophisticated neoprimitive frenzy in Le sacre du printemps; after WWI he went back to Mozart, in his fashion. Legions of composers followed each of those directions. Arnold Schoenberg blew up the last of the old harmonic system based on seven-note scales and, likewise after the war, replaced it with his own system based on the 12-tone chromatic scale. Béla Bartók drew from both Schoenberg and Stravinsky and folded those influences into his native Hungarian voice, forming a highly personal back-to-the-folk, back-to-nature kind of Modernism, in some respects so old it was new. Schoenberg's pupil Anton Webern made his teacher's system still more systematic; for many after WWII, Webern seemed to take music to a plane of ethereal intellect that formed some kind of answer to madness and conflagration. After Schoenberg died, meanwhile, Stravinsky took up his rival's 12-tone method and thereby insured its triumph in the academy if not in the concert hall. So, in music as in the other arts, the 20th century spread out in a series of contrarieties: futurism and primitivism, hyper-structure and chance, ultracomplexity and minimalism, shock-the-bourgeoisie art and Pop Art.
As of the second half of the century, Stravinsky, Schoenberg, and Bartók made up the holy trinity of musical Modernism, with Webern as chief prophet. There was a certain logic to that grouping. Schoenberg and Stravinsky formed opposing camps, the Viennese master perceived by critics as representing a rationalized, atonal, dissonant, public-be-damned aesthetic, while Stravinsky both in his neoprimitive and neoclassic veins was more sonically gorgeous, more tonal, and more communicative. (In later years, both men tried to paint themselves as traditionalists, but few paid attention.) The public embraced Bartók more slowly than it did Stravinsky, but he wrote some populist pieces (including the Concerto for Orchestra), and, however dissonant, etc., Bartók had a compelling rhythmic energy.
Webern was more removed from the mainstream than those three, but he had enormous influence on composers. John Cage was the wild card of the second half of the century. Cage unleashed, as Ross puts it, "the imp of chance" with works like Imaginary Landscape No. 4 for 12 radios (playing whatever happens to be on), and 4'33" for a pianist playing nothing for that period of time (the "music" is the squirming and annoyed muttering of the audience). Cage wanted to overthrow the aesthetics and ideologies of past centuries, and by implication the catastrophes they enabled in the 20th, by erasing the very ideas of "purpose" and "meaning" in art.
That historians settled on this group of superstars came from the critical tendency of the Modernist period to rate artists by the size of the bombs they tossed into tradition. Schoenberg and Stravinsky achieved fame via bloody audience riots; Bartók was branded a "barbarian." So, when it came time for critics and historians to judge the popular acceptance of musical Modernism, their mind-set said that innovation was the prime criterion. Schoenberg's public acceptance or lack thereof became the main litmus test for the acceptance of all 20th-century music. By that test, it hasn't succeeded too well. I've seen Schoenberg performances get standing ovations in Boston Symphony Hall, but on the whole, 57 years after Schoenberg's death, the mainstream concertgoing public remains wary of him.
Following that group of superstars, critical consensus decreed a mass of composers both radical and conservative who were in effect historical also-rans, even if they had significant influence and/or simply wrote broadly appealing music: Berg and Shostakovich, Ives and Sibelius, Ligeti and Britten, Babbitt and Reich, et al. (In some ways, Claude Debussy was the father of them all.)