When Confederate General Robert E. Lee decided to raid the Northern frontier by crossing the Potomac River, he found out that President Lincoln had replaced John Pope with George B. McClellan, as the commander of the the Army of the Potomac. General McClellan relied on two sources of intelligence to track General Lee's progress north. Some came from civilian observers, however, professional information came from the commander of his cavalry brigade, Alfred Pleasonton.
As the Battle of Antietam approached General George B. McClellan received military intelligence on the Army of Northern Virginia from three sources. One was his cavalry general Alfred Pleasonton, also from Allan Pinkerton and Maryland civilians loyal to the Union.
The private organization that Union General George McClellan hired to help gather military intelligence was the Union Espionage Organization.
Directorate of General Military Intelligence ended in 2003.
Major General George B. McClellan no longer used Allan Pinkerton as a source of military intelligence as he approached what would become the Battle of Antietam. His men had found a good source in the so-called lost order of General Lee that described Lee's battle plans. With that said, he made good use of the cavalry of General Pleasanton and received information from friendly civilians.
Clearly the so-called Maryland campaign was the climax of Major General George B. McClellan's military career and the Battle of Antietam his greatest and most important battle. The Battle of Antietam is legendary as the bloodiest day in US military history with over 25,000 casualties in one day which includes both sides of the battle.
Major General Nathaniel P. Banks reported to his commanding general in August of 1861, General George B. McClellan. Banks was in charge of the Department of the Shenandoah, with his headquarters being in Sandy Hook, Maryland. McClellan informed Banks that a Confederate offensive would begin shortly and Banks was ordered to report military operations to McClellan at least four times per day.
Upon learning of the raid into Maryland by General Lee and his Army of Northern Virginia, Major General George B. McClellan knew he was facing a challenge from the South's best general and its best army. With that in mind, McClellan began to follow the Confederates slowly, making sure that Washington DC and Baltimore, Maryland were secure. He also needed time to reorganize his Army of the Potomac in order to meet Lee's challenge.
In general terms, General George B. McClellan served only four months as general in chief. During his tenure there were several major developments. The Peninsula campaign was tried and failed, which led to the Second Battle of Bull Run. This in turn was followed by the South's march into Maryland and into the Battle of Antietam. This was yet another significant military operation. McClellan was relieved of duty some seven weeks after that horrific battle.
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.
General Meade remained loyal to his former commander, George B. McClellan, even after Meade was appointed the leader of the Army of the Potomac. He, like McClellan outwardly criticized the political interference the military had to deal with concerning military issues.
Overestimating enemy numbers, through faulty intelligence supplied to him by Allan Pinkerton.
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.