The key to understanding the American Civil War is.... Soren Kierkegaard.
The reader may be familiar with Kierkegaard's three stages of existence - the aesthetic, the ethical and the religious. I will not be concerned with the religious stage of existence in this post. What will concern me is the aesthetic and ethical stages, and specifically one essential difference between them. The ethical man is the man who lives within the concept of duty; and duty is the existential expression of one who lives within history rather than (falsely) above it or outside it. To the aesthetic man, if he has a concept of duty, it is not duty as known by a truly ethical man. The duty of the truly ethical man seems vulgar and anti-intellectual to the aesthetic man. One must first understand history and its meaning before one can know one's duty within that history, or so the aesthetic man thinks. "Duty" for him means constructing a personal theory of history and then putting himself in a relation to that history. Duty for the truly ethical man involves no speculation about history; it reflects his knowledge of what he owes his neighbor, his family, his country, and his God. The aesthetic man views himself as superior to history. He does not exist within history but is history's spectator and judge. He participates in history to the extent he judges that a positive outcome may result from his participation, but he always tempers his commitment in the knowledge that his judgment is fallible and may need to be revised.
General George B. McClellan was the commander of the Union Army of the Potomac during the critical battles of 1862; he was essentially an aesthetic man. General Robert E. Lee was the commander of the Confederate Army of Northern Virginia during much of the same time; he was essentially an ethical man. It was Lee's character as an ethical man that was the key element in his military superiority over McClellan.
As an aesthetic man, McClellan attempted to overawe history with his own personality. McClellan had his own understanding of the historical meaning of the Civil War and he "placed" himself in it. That place was, naturally, that of "Savior of the Union." Every decision McClellan made was made with one eye on the military situation and another eye on his place in history. This was the source of his famous caution. A serious military defeat, McClellan knew, could result in his dismissal, an event McClellan considered more calamitous for the Union than any military setback. Since he was history's designated Savior of the Union, what would happen to the Union without him? Obviously, the Union could only fall if McClellan were not around to save it. When Lincoln eventually did dismiss McClellan, McClellan's greatest lament was for the poor Union that would no longer have the benefit of his historically necessary services. Lincoln was right in more ways than he knew when he called the Army of the Potomac "McClellan's Bodyguard."
Lee, on the other hand, had no interest in the grand meaning of history. His decisions were all driven by his sense of duty. Offered command of all Union forces by Lincoln, Lee carefully considered the question and eventually sided with the South on the principle that his first duty lay with his home state of Virginia before it lay with the Federal government. His military decisions were driven strictly by military concerns. His disinterest in speculative history gave him a clear sight of immediate reality, and that clear appraisal of the immediate situation gave him a decisive advantage in battle over McClellan. Lee had no concern for his personal place in history; this was the source of his remarkable daring in battle. Lee never wondered whether the South could survive without him. In fact he offered his resignation after the defeat at Gettysburg in 1863.
Now this wouldn't be a Kierkegaard post without a little irony. The irony of the aesthetic man is that his obsession with universal history causes him to have no historical significance, or at least not the historical significance he desired. McClellan did not go down in history as the savior of the Union, but as someone who came close to losing it. The irony of the ethical man is that, by leaving the historical significance of his life to God and concentrating on duty, he may end up having a deep historical significance, probably one he never could have imagined.
In Lee's case, Lee was in large measure responsible for the successful reintegration of the South into the Union in the decades after the war. After the surrender of his Army of Northern Virginia, there were voices who wished to continue an indefinite guerrilla war against the Union forces, something that was very possible. In effect, the South could not win its own nation, but could nonetheless destroy the Union. Lee, with his sense of duty, quickly dismissed any such ideas. The defeat of the South was the judgment of God, Lee thought, and it was the duty of Southerners to do their best to become good citizens of the reunited Union. Lee's military exploits had made him legendary and given him a unique authority with Southerners. Only Lee had the stature to order Southerners to end their hopes of independence and become American citizens again.
The greatest irony, of course, is that Lee, in the end and in a very real sense, became the savior of the Union that McClellan always took himself to be.