What does the fog reveal as it lifts in chapter 1?

The Red Badge of Courage is a novel published in 1895 about the American Civil by the American writer Stephen Crane. The story and first chapter opens with a fog lifting to reveal "an army stretched out on the hills." The exact passage is:

The cold passed reluctantly from the earth, and the retiring fogs revealed an army stretched out on the hills, resting. As the landscape changed from brown to green, the army awakened, and began to tremble with eagerness at the noise of rumors.

The lifting fog allows the army to see what had previously been hidden to them:

It cast its eyes upon the roads, which were growing from long troughs of liquid mud to proper thoroughfares. A river, amber-tinted in the shadow of its banks, purled at the army’s feet; and at night, when the stream had become of a sorrowful blackness, one could see across it the red, eyelike gleam of hostile camp-fires set in the low brows of distant hills.

It is interesting that Crane refers to the army here as "it." As the reader finds out there is no room for individuals and individual thought in the army, and in fact the main character, Henry Fleming, a private of the Union Army, longs to be wounded (wear a red badge of courage) so he can prove his bravery to the others.
The other interesting use of words is the word retiring to describe the lifting of the fog. Once it lifts the soldiers seem full of life:

He came flying back from a brook waving his garment bannerlike. He was swelled with a tale he had heard from a reliable friend . . . To his attentive audience he drew a loud and elaborate plan of a very brilliant campaign.

It is not until the men realize the reality of the campaign that they begin to quieten down.

When he had finished, the blue-clothed men scattered into small arguing groups between the rows of squat brown huts. A negro teamster who had been dancing upon a cracker box with the hilarious encouragement of two score soldiers was deserted. He sat mournfully down. Smoke drifted lazily from a multitude of quaint chimneys.

Almost as if he wants the fog to return and again shield his eyes from the awaiting enemy, Henry goes back to his hut "to be alone with some new thoughts."
https://etc.usf.edu/lit2go/176/the-red-badge-of-courage/3196/chapter-1/

As dawn breaks and the fog dissipates, the opening scene of the story is suddenly revealed. The massed ranks of the Union army are encamped across the hills, trembling with eagerness at the imminent battle ahead. The lifting of the fog is ironic, because the fog of ignorance still remains; the soldiers are dead keen to start fighting, yet remain blissfully unaware of the horrors that lie in store.
To the likes of Henry Fleming, war is still just a big adventure, an opportunity for young men to prove their courage and manhood on the field of battle. As yet, Henry and his comrades haven't the faintest idea of what armed conflict actually involves. And though the earth has awakened with the dawn, the fog lifted, and the hills turned from brown to green, the young men encamped on those hills, chomping at the bit to start fighting, have yet to wake up to the harsh realities of war.