At the very beginning of the 20th century, Sibelius's music made a breakthrough in the Nordic countries, and also in Germany and Britain. However, time would show that his reputation in Germany was not so secure. Later in the 20th century it was the United States and Japan that became the main Sibelian strongholds, in addition to the Nordic countries and Britain. This was reflected in writings about Sibelius. In his book on Sibelius (1931), the Englishman Cecil Gray asserted that Sibelius was the greatest symphonist since Beethoven. Olin Downes, the longstanding music critic of the New York Times, considered Sibelius to be far and away the best contemporary composer. In a poll organised by the New York Philharmonic in 1935, Sibelius emerged as the most popular composer, ahead of for example Beethoven and Ravel.

Sibelius's popularity gave rise to a powerful reaction, starting from the 1930s. This was partly due to the excesses of such Sibelian "apostles" as Olin Downes, who used Sibelius as a stick to beat other contemporary composers, including Schoenberg and Stravinsky.
When in 1937 Bengt von Törne, who had received several composition lessons from Sibelius, praised his idol and spoke disparagingly of Mahler and Debussy (in his work Sibelius: A Close up), there were influential writers who felt things had gone far enough. Thus, in the 1930s Sibelius became an object of musico-political disputes.

In 1938, the German sociologist Theodor W. Adorno tore von Törne's eulogy to shreds. He found it incomprehensible that von Törne considered Sibelius a greater composer than Gustav Mahler or Arnold Schoenberg. He published his views in the column "Besprechung" of the journal Zeitschrift für Sozialforschung. The journal was published by the Institut für Sozialforschung, which had been founded in Frankfurt in 1924. To Adorno, Sibelius was no better than a "scribbler", someone "at the level of amateurs who are afraid to take lessons in composition".

In an aside, he also made a connection between the "unity with nature" that admirers had found in Sibelius's music and the "Blut und Boden" ideas of the Nazis – undoubtedly because Sibelius's popularity was on the increase not only in Britain and the United States, but also in Germany under the rule of Hitler.

The article would have been forgotten if Adorno had not risen to an enormous fame in the 1960s as one of the luminaries of the "Frankfurt School". The renowned sociologist and cultural figure returned to Sibelius in the lectures on modern music he gave at Darmstadt, citing Sibelius as a "dangerous example". As late as 1968 Adorno included the article in his collection Impromptus, under the heading Glosse über Sibelius (a marginal note on Sibelius).

Adorno's aversion to Sibelius gained influential followers. Downes's antagonist, the American critic and composer Virgil Thomson, defended European modernists and joined those who opposed Sibelius. And in an essay based on Adorno's views, René Leibowitz called Sibelius "the world's worst composer" at the time of the composer's 90th anniversary in 1955.
Such writings had an effect on a whole generation of critics in German-speaking countries and in France. Indeed, Sibelius's music was played less frequently in these countries after the Second World War. Sibelius's "fault" was that until the end of the 1950s he appeared to be the living embodiment of tonal music, which was labelled as conservative. He had not gone over to dodecaphony, which was thought to be the "right" way, or later to serialism.
At this stage, none of those who spoke against Sibelius bothered to look at the years of the compositions: the fourth symphony was thoroughly modern in 1911, and the formal concepts underlying the seventh symphony and Tapiola were revolutionary in the 1920s.

The reaction did not decrease Sibelius's popularity in Britain and Scandinavia, and Herbert von Karajan and Lorin Maazel continued to conduct his works in Germany and Austria after the Second World War. Sibelius's music started to become popular again towards the end of the 1980s, when renowned conductors such as Simon Rattle received praise for their recordings of Sibelius.

Sibelius is still used as weapon in music politics. In 2000, Antti Vihinen, the director of the Sibelius House in Lahti, dismissed Adorno's claims in his doctoral thesis, launching a fierce counter-attack: "In the light of post-colonial theory, Adorno's theories have proved to be nationalist, chauvinist and even racist," Vihinen claimed. Adopting a more moderate tone in her doctoral thesis of 2001, the German scholar Ruth-Maria Gleissner attempted to unravel the politicisation of Sibelius in Germany.

The stance of most Sibelius enthusiasts nowadays is that the composer should not be praised by denigrating other composers. In the history of music there is an important place for Sibelius – and also for Mahler, and for the Second Viennese School, i.e. Arnold Schoenberg, Anton Webern and Alban Berg. Using arguments such as these, Adorno's arguments have been rejected by the contributors to the Sibelius Companion edited by Glenda D. Goss (Greenwood Press 1996) and by the writers in Sibelius Studies (2001), published by Cambridge University Press. In a preface to the latter, Timothy Jackson and Veijo Murtomäki point out that Sibelius is a favourite of present-day scholars acquainted with the analytical methods of the influential theorist Heinrich Schenker (1869-1935). It is precisely the organic and symphonic logic of Sibelius's music that makes it such a good object for this kind of analysis. Today, people admire Sibelius's works for the very things that Adorno overlooked.

At the beginning of the 21st century Sibelius's popularity and reputation have climbed back to the level of the 1930s. When the Finnish Radio Symphony Orchestra under Jukka-Pekka Saraste played the symphonic cycle in the Konzerthaus, Vienna, in the spring of 2002, the concerts were sold out. The critics were unanimous concerning Sibelius's standing -even in this city, which had been perhaps the last classical music metropolis to reject Sibelius. "Sibelius was a modern composer in an exciting way," wrote Peter Vujica in Der Standard. The newspaper Wiener Zeitung, for its part, considered Sibelius to be a landmark in modernity.