But in June 2012 he extended the hand of friendship to the Queen, formally ending centuries of hatred.
That historic moment involving Stormont's late Deputy First Minister set the seal on a new relationship, a process that King Charles confirms today with his first official visit to Belfast.
Sinn Fein politicians, who once spoke of winning with a ballot box in one hand and an Armalite rifle in the other, will meet with him before a service at St Anne's Cathedral.
Ten years have elapsed since the unthinkable was acted out in a side room of Belfast's Lyric Theatre.
Holding the sovereign's hand for a few seconds, McGuinness spoke to her in Gaelic, explaining his words meant "Goodbye and Godspeed".
The Queen had already paved the way for this epoch-making event with a state visit to Dublin in 2011, when McGuinness denounced her trip as "premature" and refused invitations to attend ceremonies.
But the visit was popular, and with 77% of Irish people approving, Sinn Fein changed its tune.
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