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Two years ago, James Harris Jackson, a white Army veteran, fatally stabbed a man on a dark New York City street solely because he was black.

“The racial World War starts today,” Mr. Jackson wrote in a manifesto that included an emblazoned swastika and a Crusader’s Cross. “This political terrorist attack is a formal declaration of global total war on the Negro races.”

On Wednesday, Mr. Jackson, who was about to be sentenced to life in prison without the chance of parole, attempted to rewrite that history, offering an apology.

“It never should have happened, and of course, if I could do it all over again this never would have happened for sure,” Mr. Jackson said.