Won't do to be too pessimistic, and say the songs have no power. When the powers that be decide these things, it is often a finely balanced decision. Politicians will balance the likely benefits to the world(or themselves) of going to war, against the undoubted miseries. Sometimes it goes one way, sometimes the other.
    And songs of peace undoubtedly reinforce our awareness of the horrors of war. So it is a fairly safe assumption, that whenever a group of people decide on a peaceful course of conduct, they will have done so partly because of the influence of song. So, what I mean is, don't dwell on the wars the songs failed to stop: think about the wars that didn't happen, that songs helped to stop. And keep singing.