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I agree with BernieO above, that violence only begets violence, but to my mind what destroyed the peaceful anti-war movement was the murder of four students at Kent State by the national guard, with impunity. As a military veteran, I know full well that the explanations given were sheer poppycock, i.e. no one knew their guns had live ammunition and no one knew who gave the order to shoot. When live munitions are issued, the military makes a point of apprising everyone of the potential for danger, and anyone who shoots a weapon without orders to do so, especially domestically, is subject to a court martial. All military groups know who is in charge, and everyone in the military knows when his gun is loaded with live rounds. Once you realize that the federal government is willing to kill college kids on campus, it's a whole new ball game.
Then, there were the murders of JFK, MLK, RFK, the freedom riders in Dixie, the Black Panthers in Chicago, the Chicago police riot, inter alia. As others have pointed out, little kids were being roasted alive by Napalm, and Vietnamese children are still being born with horrendous defects due to agent orange. It's hard to continue life as usual when such monstrosities are being committed.