Luck? Skin of our teeth? Sounds like a football game. There was no single event or seminal moment that determined the out come of the Civil War. It was a long, grinding war of attrition that the Confederacy never could have won. The Union advantage in numbers of men, industrial and military capacity were overwhelming from the beginning. The naval and economic blockade stangled the south. And Sherman's march to the sea was something more than the mere application of brute force. All these historical facts are more than evident.
The "total war" theory is what eventually won the war. However, if the Confederates could have captured and cut off Washington the long war of attrition that the Civil War eventually became would never have occurred; support for the war in the Northern states, especially in Northern cities like New York, was already low and a huge symbolic loss such as the loss of the nation's capital would have almost certainly caused the North to start looking for a peaceful resolution - in other words, surrender.
The naval blockade did severely weaken the Southern economy, but it alone wouldn't have won the war. Without Sherman and Sheridan, the true destruction of the southern economic base never would have occurred - and neither of those two men got a shot at being generals until after Gettysburg. Without the narrow defeat of the Confederacy at Antietam, or the defeat of the Confederacy at Gettysburg which happened due to Lee's biggest tactical blunder, the "war of attrition" never would have occurred.
But, I guess some people like to think that every point of view deserves equal consideration. As if every trivial notion merits some credibility. And of course there are those who want to accept any idea that supports what ever mindless lunacy they are pre-disposed to believe. So, your shallow analysis and dubious conclusions are not really surprising.
Oh, the arrogance. The world just lays down at your feet, doesn't it?
All views, especially strongly-held ones, deserve consideration. German leaders in the 20s dismissed out of kind an angry little extremist splinter group, believing it to be too small and insignificant to ever amount to anything. Eventually the Nazi Party grew until distinguishing it from the German government became nearly impossible.
Tell me, why is it shallow to reject the idea of our infallibility in war, especially given the present situation of American armed forces in Iraq and that the South in the Civil War possessed far greater generals, which made up for what they lacked in industrial might and manpower? Seems to me that brushing aside the Civil War as a minor conflagration that the South never had a hope of winning is the shallow line of reasoning.
Let's compare the American Revolution to the Civil War, shall we?
Who won the Revolution, the yokels and farmers with homemade weapons led by brewery owner or the most well-trained, well-equipped army in the world? The yokels did, despite being outclassed, out-manned, and out-gunned.
They won by superior usage of tactics, picking their battles well, using foreign aid, and wearing down the enemy to the point where they just gave up.
Tactics - The American patriots in the Revolutionary War used guerrilla-style tactics, rather than traditional European-style tactics, which allowed them to hide out and take down British units while the Brits were caught standing up. Southern generals also used superior tactics during the Civil War - despite being up against superior forces in all major conflagrations of the war, they won using an array of superior tactics. Chancellorsville, Fredericksburg, and both battles of Bull Run are all excellent examples.
Picking their battles - American revolutionary forces knew that if they stood and confronted a British field army head on they'd probably get massacred. Therefore, General Washington chose his battles carefully, only attacking when chances were best of victory. The South did this as well - except, notably, at Gettysburg.
Foreign aid - It is quite probable that American forces would not have won the Revolutionary War without the intervention of France, which provided us with their Navy so that we could launch what turned out to be the last battle of war - Yorktown. During the Civil War, both Britain and France, who were economic partners with the South (using Southern cotton in their textile factories) were just itching to recognize the Confederacy as an independent nation, which would have directly led to their intervention in the war on the side of the South. The only thing that prevented them from doing so was the Emancipation Proclamation - and the only reason Lincoln managed to get the thing out was Antietam. Without a Union victory at Antietam, Britain and France probably would have recognized the Confederacy as an independent nation and done their part to end hostilities.
Wearing down the enemy - Countries often find it difficult to fight unpopular wars. After years of trying to subjugate the American forces during the Revolutionary War, the British just gave up. Don't delude yourself into thinking that we "beat" them; they gave up, even though they could, potentially, have thrown enough divisions at North America to wipe us off the map. The same could easily have befallen the North in the Civil War; without victory to show that he was at least winning the war, Lincoln was becoming more and more despised by Northern citizens. Without victory at Antietam, holding back the growing masses of war critics would quite probably have gone beyond Lincoln's control.
As you can see, the whole thing hinges on Antietam. Without a Union victory there, Southern forces would have been able to surround and cut off the city, a major symbolic and tactical blow to the United States. Britain and France would have recognized the Confederacy as an independent nation and put their considerable influence behind ending the war in favor of their economic partner. Northern citizens already discouraged with the two-year, perpetually negative war would have expressed a total lack of support in carrying the thing onwards.
Now, if I'm being so shallow, perhaps you'd like to point out how the Union would have managed to come back from a loss at Antietam? That is, the loss that probably should have happened, if not for that courier's blunder.