This article: Over the years, Nielsen wrote the music for over 290 songs and hymns, most of them for verses and poems by well-known Danish authors such as N. F. S. Grundtvig, Ingemann, Poul Martin Møller, Adam Oehlenschläger and Jeppe Aakjær. In Denmark, many of them are still popular today both with adults and children. They are regarded as "the most representative part of the country's most representative composer's output". In 1906, Nielsen had explained the significance of such songs to his countrymen:With certain melodic inflections we Danes unavoidably think of the poems of, for example, Ingemann, Christian Winther or Drachmann, and we often seem to perceive the smell of Danish landscapes and rural images in our songs and music. But it is also clear that a foreigner, who knows neither our countryside, nor our painters, our poets, or our history in the same intimate way as we do ourselves, will be completely unable to grasp what it is that brings us to hear and tremble with sympathetic understanding.
Of great significance was Nielsen's contribution to the 1922 publication,  Folkehøjskolens Melodibog (The Folk High School Songbook), of which he was one of the editors together with Thomas Laub, Oluf Ring and Thorvald Aagaard. The book contained about 600 melodies, of which about 200 were composed by the editors, and was intended to provide a repertoire for communal singing, an integral part of Danish folk culture. The collection was extremely popular and became embedded in the Danish educational system. During the German occupation of Denmark in World War II, mass song gatherings, using these melodies, were part of Denmark's "spiritual re-armament", and after the war in 1945 Nielsen's contributions were characterised by one writer as "shining jewels in our treasure-chest of patriotic songs". This remains a significant factor in Danish assessment of the composer. contains an answer for the question: What major world event led to Denmark seeing the man who wrote over 290 songs as someone who created "shining jewels in our treasure chest", what is it ?


World War II