Something forbade me either to enter or retire, (a) a feeling. i know not how it
came that i was in the presence of imminent tragedy and (b) might serve my friend
by remaining
b.

He was worried that he might die soon in battle and wanted to communicate his love for Sarah.

Explanation:

Sullivan Ballou wrote a letter to his wife Sarah to say goodbye in case he died in the war. We can see that he is a soldier fighting for the independence of the USA, since he shows in the letter that he is not sorry to go to the fight, because he feels honored to fight for the country. However, we can see that the real reason for the letter is to show Sarah how much he loves her and how much he loves his children. This is shown from the beginning of the letter, when he says "Lest I should not be able to write again, I feel impelled to write a few lines that may fall under your eye when I shall be no more."

He also states that if the dead can return to the land, he will always stay close to her and the children, watching and loving them.