It is not too much of a stretch to say that there was more at stake during the first two weeks of September 1862 than at any other time during the war.
Southern armies were on the move across in Mississippi, Kentucky, and western Virginia. In Virginia, after defeating two Union armies, Robert E. Lee had transferred the front from the gates of Richmond to the gates of Washington. A triumphant Army of Northern Virginia crossed into Maryland on September 4. The Confederacy would never be closer to winning the war.
Defeated and demoralized, two Union armies took refuge in the Washington defenses. "It was indeed humiliating," one Union soldier wrote. "Here we were, after six months of campaigning, back again at the point where we started." It can be argued that, during a war with many low points, this may well have been Abraham Lincoln's lowest. It was during these days that the President composed his famous "Meditation on Divine Will."
In this crisis, Lincoln turned to George B. McClellan. During the past few weeks, McClellan had essentially had his command removed from him, but after Pope's disastrous defeat at Second Bull Run Lincoln believed McClellan was only man capable of meeting the crisis. "If he can't fight himself," Lincoln said, "he excels in making others ready to fight." On September 5, Lincoln ordered McClellan to take the field to stop Lee's invasion.
Whether you like McClellan or hate him, what he accomplished over the weeks was pretty amazing. His organizational skills were put to full service as he molded an army to take the field from five separate organizations -- 3 corps from the old Army of the Potomac (one of which had suffered heavy casualties at Bull Run), 2 corps from the ill-fated Army of Virginia (including one nearly decimated at Cedar Mountain), 2 divisions from Burnside's Expeditionary force, 1 division from South Carolina, and 1 from western Virginia. To this mix he added in 18 rookie regiment's, many of whom had not even had the most basic drill, and some of whom loaded and fired their weapons for the first time on the march.
McClellan fought two major battles, during both of which he ordered his men to attack, and ultimately forced Lee to retreat back to Virginia. This combined with Buell's victory at Perryville, essentially ended the great crisis of September 1862.
On September 18, McClellan took a moment to write his wife, informing hwr of the Battle of the day before. The famous passage from this letter is, "Those on whose judgment I rely tell me that I fought the battle splendidly and that it was a masterpiece of art." The rest of the passage indicates the strain McClellan was under: "The spectacle yesterday was the grandest I could conceive of -- nothing could be more sublime.... I am well nigh tired out by anxiety and want of sleep."