Almost since the dawn of humanity, there's been conflict and almost since the dawn of conflict individuals have asked why?  At the lowest, individual level in a trench & facing an enemy's sword just beyond a small mound of earth, what's the last motivation that drives a soldier to fight?   A rash of recent movies (Saving Private Ryan, Blackhawk Down, etc.) have argued that what pushes men past the immediacy of self preservation was group bonding.  You fought to do your duty to the soldier next to you.  

While I agree at some level, I've never been really satisfied with this answer and find it far from complete.   I particularly dislike the almost Orwellian / group-think flavor our modern cynics are oh-so-quick to find comfort in. 

Others - like Victor Davis Hanson - argue instead that it's a strongly internalized mission - each soldier is a statesman.   Duty, honor, and purpose rise into a calling which many civilians can scarcely understand but which are inestimably important for the comforts we blithely enjoy.   That's certainly the sentiment here - a profoundly stirring letter written by a civil war soldier to his wife on the eve of battle questioning his motives and explaining the risks he knowingly subjects her and his family to -

July the 14th, 1861

Washington D.C.

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 you again, I feel impelled to write lines that may fall under your eye when I shall be no more.

Our movement may be one of a few days duration and full of pleasure -- and it may be one of severe conflict and death to me. Not my will, but thine 0 God, be done. If it is necessary that I should fall on the battlefield for my country, I am ready. 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 upon the triumph of the Government, and how great a debt we owe to those who went before us through the blood and suffering 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.

But, my dear wife, when I know that with my own joys I lay down nearly all of yours, and replace them in this life with cares and sorrows -- when, after having eaten for long years the bitter fruit of orphanage myself, I must offer it as their only sustenance to my dear little children -- is it weak or dishonorable, while the banner of my purpose floats calmly and proudly in the breeze, that my unbounded love for you, my darling wife and children, should struggle in fierce, though useless, contest with my love of country?

I cannot describe to you my feelings on this calm summer night, when two thousand men are sleeping around me, many of them enjoying the last, perhaps, before that of death -- and I, suspicious that Death is creeping behind me with his fatal dart, am communing with God, my country, and thee.

I have sought most closely and diligently, and often in my breast, for a wrong motive in thus hazarding the happiness of those I loved and I could not find one. A pure love of my country and of the principles have often advocated before the people and "the name of honor that I love more than I fear death" have called upon me, and I have obeyed.

Sarah, my love for you is deathless, it seems to bind me to you 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 irresistibly on with all these chains to the battlefield.

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 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 grow 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 battlefield, it will whisper your name.

Forgive my many faults, and the many pains I have caused you. How thoughtless and foolish I have oftentimes been! How gladly would I wash out with my tears every little spot upon your happiness, and struggle with all the misfortune of this world, to shield you and my children from harm. But I cannot. I must watch you from the spirit land and hover near you, while you buffet the storms with your precious little freight, and wait with sad patience till we meet to part no more.

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 garish day and in the darkest night -- amidst your happiest scenes and gloomiest hours -- always, always; and if there be a soft breeze upon your cheek, it shall be my breath; or 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.

As for my little boys, they will grow as I have done, and never know a father's love and care. Little Willie is too young to remember me long, and my blue-eyed Edgar will keep my frolics with him among the dimmest memories of his childhood. Sarah, I have unlimited confidence in your maternal care and your development of their characters. Tell my two mothers his and hers I call God's blessing upon them. O Sarah, I wait for you there! Come to me, and lead thither my children.

Sullivan

If you can read it with a dry eye, you're either a stronger or more foolish man than I.

UPDATE:   Why they fight:

...[People who don't understand volunteer combat soldiers have] a common cynical, or even nihilist assumption which I do not share: that intelligent people do not seek to serve in a volunteer military, or that if they do, they fail to seek challenging and dangerous assignments.

...Rather, I would suggest that when confronted with a totalitarian regime that practices systematic torture, rape, genocide, and murder, and which actively supports terrorist organizations which make a practice of setting off bombs on school busses abroad—and when offered a chance to help contribute to its demise—a truly intelligent man would discern a certain moral obligation.