Celebrated for its fragile simplicity and emotion, the "Adagio" might have seemed an odd match for Toscanini, known for his power and drama as a conductor. But according to Mortimer Frank, author of Arturo Toscanini: The NBC Years, despite the director's force and intensity, he was capable of "wonderful delicacy and tenderness and gentleness."
The year 1938 was a time of tumult. America was still recovering from the Depression and Hitler's Germany was pushing the world towards war. Toscanini himself had only recently settled in America after fleeing fascist Italy. The importance of the broadcast performance during this time is noted by Joe Horowitz, author of Understanding Toscanini: "Toscanini's concerts in New York... once he was so closely identified with the opposition to Mussolini, the opposition to Hitler — these were the peak public performances in the history of classical music in America. I don't think any concerts before or since excited such an intense emotional response, and I don't think any concerts before or since evoked such an intense sense of moral mission."
The "Adagio for Strings" was written by American composer Samuel Barber when he was in his 20s. With a tense melodic line and taut harmonies, the composition is considered by many to be the most popular of all 20th-century orchestral works.
"You never are in any doubt about what this piece is about, says music historian Barbara Heyman. "There's a kind of sadness and poetry about it. It has a melodic gesture that reaches an arch, like a big sigh... and then exhales and fades off into nothingness."
In January of 1938 Barber sent the piece to Arturo Toscanini. The conductor returned the score without comment, and Barber was annoyed and avoided the conductor. Subsequently Toscanini sent word through a friend that he was planning to perform the piece and had returned it simply because he had already memorized it. It was reported that Toscanini did not look at the music again until the day before the premiere.  The work was given its first performance in a radio broadcast by Arturo Toscanini with the NBC Symphony Orchestra on November 5, 1938 in New York.
The composer also transliterated the piece in 1967 for eight-part choir, as a setting of the Agnus Dei ("Lamb of God"). It has since become renowned as a masterwork of the modern choral repertory.
The long, flowing melodic line moves freely between the voices in the string choir; for example, the first section of the Adagio begins with the principal melodic cell played by first violins, but ends with its restatement by violas, transposed down a fifth. Violas continue with a variation on the melodic cell in the second section; the basses are silent for this and the next section. The expansive middle section begins with cellos playing the principal melodic cell in mezzo-soprano range; as the section builds, the string choir moves up the scale to their highest registers, culminating in a fortissimo-forte climax followed by sudden silence. A brief series of mournful chords serve as a coda to this portion of the piece, and reintroduces the bass section. The last section is a restatement of the original theme, with an inversion of the second piece of the melodic cell, played by first violins and violas in unison; the piece ends with first violins slowly restating the first five notes of the melody in alto register, holding the last note over a brief silence and a fading accompaniment.