Carl Nielsen is widely regarded as Denmark's greatest composer, and this is perhaps his greatest composition. Written in 1916 at the height of the War, it is deeply influenced by that conflict. It was unusual for a symphony in that the four movements are played without breaks - in essence, it's one long non-stop battle (in fact, it is scored attacca subito: attack suddenly). In the final movement a pair of tympanis duel from opposite sides of the stage in a mock artillery exchange.
Outside of Denmark, Nielsen is known for compositions like this: orchestral music. In Denmark he is much more of a national symbol, and his songs (influenced by traditional Danish folk songs) are more widely known. During the Nazi occupation in World War II his songs were often performed as an act of resistance.