Meanwhile, Harry and St.Clair were descending the mountain, pausing now and then to survey the valley with their glasses, where they could yet mark the movements of the Northern troops.

When they reached the cove they found that the board and the chess men were put away, and the two colonels were inspecting the Invincibles to see that the last detail was done, while Early made ready for his desperate venture.
Harry and his comrades were fully conscious that it was a forlorn hope.
They had been driven out of the valley once by superior numbers and equipment, directed by a leader of great skill and energy, but now they had come back to risk everything in a daring venture.

The Union forces, of course, knew of their presence in the old lines about Fisher's Hill-- Shepard alone was sufficient to warn them of it--but they could scarcely expect an attack by a foe of small numbers, already defeated several times.
Harry's thought of Shepard set him to surmising.

The spy no longer presented himself to his mind as a foe to be hated.

Rather, he was an official enemy whom he liked.