The Zombies' "A Butchers Tale (Western Front 1914)." Still, for my money, the most powerful anti-war song of them all.
Actually, I remember where I was on this day in 1968. Vividly. And I can guarantee that if you had told me that, forty years later, the United States would be once again involved in another abominable, immoral war in a country on the other side of the world that had not attacked us, I would have assumed you were doing drugs far more potent than any I had yet encountered.