"Threw up a yellow hare /
and crawled in a threshing circle"
The hare could be a metaphor for the soldier as it has been caught up in the conflict, like the soldier, where the 'circle' is a metaphor for war, suggesting that there is no start or end to it.
The hare is continuously running, like the soldier, just because it wants to survive. Hughes uses the hare as a symbol of the soldier's collective suffering, the sense of panic & fear.
The colour yellow is also associated with cowardice and Hughes does this to show symbolism for the soldier's emotions.