John Keegan, the renowned British military historian, writes: “The American Civil War is one of the most mysterious great wars of history, mysterious because unexpected, mysterious also because of the intensity with which it took fire.” Among the war’s mysteries, he lists the following:
Why should men who lacked a rational interest in the war struggle so fiercely against Northerners who … were frequently not to be distinguished from their poor Southern opponents?
After all, Keegan notes, “many Southerners had no personal connection with slavery at all, neither as owners of slaves nor as employers of their labour. The considerable slave owners were … often resented by their non-slave-owning neighbours, though that did not deter them from joining in their thousands in the new Confederate army.”
This lack of “direct personal motivation” was an apparent “paradox,” which emphasized an “undeniable fact”:
The gray ranks were but sparsely populated with large slave owners or their sons, but enormously by hardscrabble farmers and often by men who owned nothing at all.
Shelby Foote, author of The Civil War: A Narrative (3 vols.) offers an explanation for the motive of the Confederate soldier in Chapter Three (“Forever Free”) of Ken Burns’ documentary The Civil War:
The answer a Southerner would give you as to “why are you are fighting?” if you were a Northerner, he would say: “I’m fighting because you’re down here.” He was being invaded and he thought … to defend his home.
“Because you’re here” seems both a legitimate reason to fight a war, and also a sound precept for preventing one. It illuminates the true nature and folly of many of our historic conflicts.
This is always the answer given by national insurgents to imperialists and colonialists: We fight because you’re here. And this is always the simple motivation invaders never get, the plain fact that people who have been fed a steady diet of “saving the world for democracy” or “Operation Iraqi Freedom” slogans never understand, or are willing to accept.
No doubt there are other reasons why the Algerians fought the French in Africa, why the Irish and the Indians in Asia fought the British, why the Afghans fought the Russians, and why the Vietnamese fought against us in Indochina. But in every case, when the invader is no longer there, the war stops.
 John Keegan, The American Civil War: A Military History (New York: Alfred A. Knopf, 2009), xv-xvi.