Why was harriet tubman successful as a spy for the union during the civil war

Tubman and other former slaves were effective as spies because white Confederates devalued their intelligence. “They had lived their lives as invisible people,” writes Allen in his book. “That quality of invisibility, which Harriet Tubman knew so well, became the basis for using ex-slaves as spies for the Union.”

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a term is used to describe a sudden revelation in a short story is "showing forth".