Answer: A) It sets up a contrast between "To err" and "to forgive" and B) It helps the speaker emphasize the idea of forgiveness.
Explanation: In poetry, a caesura is a pause in a line that is formed by the rhythms of natural speech rather than meter. A caesura will usually occur in the middle of a line of poetry but can occur at the beginning or the end of a line. In the given excerpt the use of the caesura in the last line, shows contrast between to err and to forgive and it helps the speaker emphasize the idea of forgiveness.
It appears that the author has intended to illustrate how it was back then compared to the present. The author appears to be implying that military service has lessened over time. However I do not know which essay you are reading from (you have not mentioned the name) so I am not entirely sure.
