"Where Have All The Flowers Gone?" is a folk song by Pete Seeger, written and recorded in 1955. It is based on lines from older Cossack songs, which is one of the reasons I am thinking about it now. It was recorded by many other artists in the 1950s, 1960s, and later, including Peter, Paul, and Mary and Johnny Rivers. Decades later, Green Day would rewrite it in Letterbomb. It has a long currency.
The song is perhaps an antiwar song or perhaps a melancholy song about the circle of life and death. Girls pick flowers, girls marry boys, boys become soldiers, soldiers die in war, filling graveyards, flowers grow on the graveyard... it isn't exactly metaphorically subtle.
As with much of folk music from the period, it can come across as very smug when the singers ask rhetorical questions like "When will they ever learn?". One of the reasons I prefer the Johnny Rivers version is that he turns a "heavy message" into a tragicomic irony. But of course, with dozens of versions of the song, there is probably a version for everyone's tastes.