Ben Watson is pretty much on the money here when he writes (in an antidote to most of the half-digested theory and rad-posturing drivel written about war and music of late):
Edgard Varèse brought the noise of sirens and bombs into music in the 1920s, a response to the terrors of World War I. His Hyperprism predicted the Nazi strategy of the Blitz, when civilian populations first became long-distant targets of military hardware. Unlike his ‘objectivist’ follower Iannis Xenakis, Varèse bent the shapes he heard into organic ovaloids which speak for the suffering ear. This is why, of all the pre-war orchestral composers, only Varèse has a non-salon, yet humanist ruggedness: a realism that moves the blood and shakes the entrails. Sonically, Varèse can stand comparison to Coltrane and Hendrix, who provided lasting testimonials to a different noise: a struggle against racial oppression in America and genocidal war in Vietnam.