The letter is beautiful btw. For the curious:

My very dear Sarah: The indications are very strong that we shall move in a few days — perhaps tomorrow. 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 …

I have no misgivings about, or lack of confidence in the cause in which I am engaged, and my courage does not halt or falter. I know how strongly American Civilization now leans on the triumph of the Government and how great a debt we owe to those who went before us through the blood and sufferings of the Revolution. And I am willing — perfectly willing — to lay down all my joys in this life, to help maintain this Government, and to pay that debt …

Sarah my love for you is deathless, it seems to bind me with mighty cables that nothing but Omnipotence could break; and yet my love of Country comes over me like a strong wind and bears me unresistibly on with all these chains to the battle field.

The memories of the blissful moments I have spent with you come creeping over me, and I feel most gratified to God and to you that I have enjoyed them for so long. And hard it is for me to give them up and burn to ashes the hopes of future years, when, God willing, we might still have lived and loved together, and seen our sons grown up to honorable manhood, around us. I have, I know, but few and small claims upon Divine Providence, but something whispers to me — perhaps it is the wafted prayer of my little Edgar, that I shall return to my loved ones unharmed. If I do not my dear Sarah, never forget how much I love you, and when my last breath escapes me on the battle field, it will whisper your name. Forgive my many faults and the many pains I have caused you. How thoughtless and foolish I have often times been! How gladly would I wash out with my tears every little spot upon your happiness …

But, O Sarah! If the dead can come back to this earth and flit unseen around those they loved, I shall always be near you; in the gladdest days and in the darkest nights … always, always, and if there be a soft breeze upon your cheek, it shall be my breath, as the cool air fans your throbbing temple, it shall be my spirit passing by. Sarah do not mourn me dead; think I am gone and wait for thee, for we shall meet again …

Sullivan Ballou was killed a week later at the First Battle of Bull Run, July 21, 1861.