Despite having at least three sources of military intelligence at his disposal, all of them over estimated the size of Robert E. Lee's army in Maryland. Four days before the US Civil War Battle of Antietam, McClellan sent a telegram to General in Chief Henry W. Halleck that he believed Lee had 120,000 troops. On the single day of battle near Sharpsburg, the Order of Battle for the Army of Northern Virginia was 39,000. McClellan had 70,000 troops. There seems no rational explanation as to why McClellan's report to Halleck informs Halleck that Lee had 120,000. Surely he knew his troop strength, and if so, his 70,000 man army would have no chance of winning any sort of battle with the Army of Northern Virginia.
As Major General George B. McClellan was following the Confederate army in Maryland, he sent to Washington DC on September 10, 1862, that the Rebel forces numbered 120,000 soldiers. This was a gross error as Lee had less than 50,000 troops. McClellan's source of information on this was from a civilian observer.
General McClellan aimed to block General Lee at the crucial location of Antietam Creek during the Maryland Campaign. He recognized that the Confederate Army was divided, with part of Lee's forces stationed in Virginia and the other near Sharpsburg, Maryland. By engaging Lee at Antietam, McClellan sought to exploit this division and deliver a decisive blow to the Confederate forces. This strategic opportunity ultimately led to the bloodiest single-day battle in American history on September 17, 1862.
Union General George B. McClellan received the battle plans for the Confederate invasion of Maryland on September 13, 1862. He recognized the value of knowing Lee's plans and the opportunity he had to catch and defeat Lee. McClellan immediately informed President Lincoln of this opportunity.
On September 14, 1862, General George B. McClellan ordered General Franklin to advance his corps toward the town of Harper's Ferry as part of the Union's efforts during the Maryland Campaign. McClellan sought to secure the strategic location and cut off Confederate forces. Franklin's orders emphasized a swift movement to support the broader objectives of the Union Army in the area. This maneuver was crucial in the context of the approaching Battle of Antietam.
General McClellan was aware of how President Lincoln had used troops to secure the border slave state of Maryland. McClellan was also aware that the port of Baltimore could be a Confederate target. With that in mind he recommended in 1861 that 5,000 troops should be garrisoned in Baltimore.
On September 16, 1862, one day before the historic Battle of Antietam, Major General George B. McClellan lost his best opportunity to defeat General Lee's army in detail in Maryland. McClellan was slow to assemble his forces in and around the town of Sharpsburg, Maryland. McClellan was unaware that he substantially outnumbers Lee's army and postpones an attack on the Confederates. Instead he waits another day until his reserve forces joins his main army. Theoretically, if McClellan conducted an assault on September 16, the Battle of Antietam would have been a smashing Union victory.
Most historians cite September 17, 1862 as the bloodies day of fighting in US history. This was during the Battle of Antietam. Confederate General Lee had brought his army into Maryland where he fought the Union forces of Union General George B. McClellan.
Allan Pinkerton was hired by US General George B. McClellan for espionage work for confederate operations.
The Battle of Antietam took place near the Antietam Creek, near Sharpsburg, Maryland in September of 1862. This was the first time Confederate General Lee took his forces into Union territory. Maryland was a slave state that remained in the Union. It was one of the bloodiest battles in American history. Union General Burnside, under the command of General George B. McClellan forced Lee back into Virginia as a result of the battle. Not long afterwards US President Lincoln visited General McClellan to see the battlefield itself.
General McClellan was pleased by the change of Confederate command due to the wounded General Johnston. McClellan considered Lee to be meek and clearly was not chosen to lead any major Southern armies. Also, McClellan had a brief encounter with some of Lee's soldiers who failed to retain what would become West Virginia.
Captain George Custer was on a scouting mission for General George B. McClellan on September 15, 1862. He reported to the general that he had observed the Confederate army in full view and in perfect line, established on a ridge near Sharpsburg. Custer noted that the Rebel battle line was one and a half miles long and that Confederate General Longstreet had forty cannons.
Confederate General Robert E, Lee and Union General George McClellan