Go look up the concept behind "nearly." The Federal Army was a bumbling shamble in the first two years of the war. They let the Confederates escape after Second Bull Run due to sheer incompetence, which repeated itself after Gettysburg. They were practically broken at Fredericksburg and Chancellorsville, again due to incompetence. The South started the war with far larger numbers of experienced field officers, which gave them a huge advantage that they readily exploited many times, especially early on. Unfortunately, due to lack of supplies and a generally terrible Congress, they lost.
Even then, my point still stands. It's happened before, no reason why it shouldn't happen again. You'd think that with his push on healthcare that Obama would look into making mental healthcare a more sought-after solution. Instead of banning guns and making the sane, law-abiding citizen take punishment for the actions of an obviously deranged person, why not use those resources to help these people instead of leaving them to stew in their insanity until they snap?