Muhammad Ali's refusal to sign his Vietnam-era military draft card upended the boxing champ's life and added a powerful voice to the anti-war movement. Now that piece of history is coming up for sale.

There's a blank line on the card where Ali was supposed to sign in 1967 but refused to do so — a polarizing act of defiance as the Vietnam War raged on. It triggered a chain of events that disrupted his storied boxing career but immortalized him outside the ring as a champion for peace and social justice.

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