"You could call it divine intervention, you could call it providence, you can call it chance, you can call it circumstance, you call it the roll of the dice," said McCullough. "In any event, whatever it's called, it was out of anyone's control and absolutely, it was the decisive element again and again."
The two-time winner of the Pulitzer Prize for biography was speaking on "Fox News Sunday" to promote his new book "1776."
He cited an incident in which Washington was trapped at Brooklyn Heights, New York -- and saved when a fog bank covered the retreat of his troops.
"The wind dropped, it was like the parting of the waters of the Red Sea," said McCullough.
Washington's force of 9,000 men -- along with all their equipment, cannons and horses -- crossed the East River in the dark of night. McCullough said the colonists didn't lose a single man in during the retreat.
"It was brilliant," he said.