Dunno about that, Aeola. Sure the songs are very often written after the event, but many songs helped create motivations before events happened. Phil Ochs, Bob Dylan, Barry McGuire, Buffy Ste. Marie and many others wrote and recorded anti-war songs during the Vietnam days, helping to raise consciousness and eventually end the war. "There Were Roses" certainly made a lot of non-Irish understand the human tragedy of The Troubles, which may have contributed to their cessation. And on a much smaller level, song has traditionally been used in courtship and wooing, resulting in not a few marriages and births.

And lest we forget, music has also been used negatively, to rouse destructive impulses. Hitler certainly understood that.