And indeed, after two days of searching the horizon in vain, on the third day Aragorn spied Gandalf astride a white horse cast in a pearly glow, leading the Riders of Rohan down upon the orcish rear, peeling them away from the seige and destroying them.

Now, Tolkien was a scholar of the era and so could not have been ignorant of a certain passage in Gregory of Tour's History of the Franks.