In line five, "fires on the horizon" is most likely a metaphor for:

Sunrise and peace
Piles of burning books
Armies' campfires
Wartime's constant threat of violence

Wartime's constant threat of violence

To determine the most likely meaning of the phrase "fires on the horizon" in line five, we can analyze the given options and the context provided.

1. Sunrise and peace: This option suggests that the phrase is a metaphor for a peaceful sunrise. However, the word "fires" typically conveys a sense of destruction or danger, which contradicts the idea of peace.

2. Piles of burning books: This option suggests that the phrase refers to piles of books being set on fire. While it is possible, the phrase "fires on the horizon" is more commonly associated with larger-scale events rather than the burning of books.

3. Armies' campfires: This option suggests that the phrase symbolizes the campfires of armies. This interpretation aligns with the use of the word "horizon," as armies setting up campfires would likely be visible on a distant horizon.

4. Wartime's constant threat of violence: This option suggests that the phrase represents the constant threat of violence during wartime. The word "fires" may symbolize the literal fires caused by battles or the metaphorical fires of conflict and destruction. This interpretation captures the ominous tone conveyed by the phrase.

Given the context, the most likely metaphor for "fires on the horizon" in line five is the constant threat of violence during wartime. This interpretation corresponds to the mention of "wintertime" and the overall theme of the poem, which explores the destructive nature of war.

Based on the given options, "fires on the horizon" is most likely a metaphor for wartime's constant threat of violence.