Because he hoped to persuade Yankee politicians that the Confederates intended to fight on in the hope of obtaining a political solution instead of continued warfare.
General Lee has his cavalry ford the Potomac River to screen his main army's advance to Frederick, Maryland. This advance begins on September 4, 1862.
On September 8, 1862, General Robert E. Lee was headquartered near Fredericktown, Maryland. He issued his proclamation to Maryland's citizens carrying the theme that the Federal government had violated their Constitutional rights. He cited the unlawful arrests and abuse the US government had laid upon the people of Maryland. Lee promised the people of Maryland a return to their rights that the US government had taken away from them unlawfully.
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After the Battle of Antietam in September of 1862, Lee's Army of Northern Virginia had to retreat from Maryland back to Virginia. Lee's evacuation from Maryland, for that moment of the war ended Confederate plans to take the war to the enemy, the Union.
In early September of 1862 Confederate General Robert E. Lee had two options when his army reached Leesburg, Virginia. From that town, Lee could move his forces against Union forces in the northern end of the Shenandoah Valley and Harper's Ferry, or cross the Potomac River to the west of Washington DC and invade Maryland. His decision was to invade Maryland. This was an eventful move as the month of September moved along.
Lee had been appraised of the military situation in Maryland on September 15, 1862. He therefore ordered generals Longstreet and Hill to take defensive positions at Sharpsburg, Maryland. Lee also ordered generals Jackson and McLaws to arrive to Sharpsburg as soon as possible. Lee was injured and using an ambulance to survey where the battle of Antietam would be fought.
Antietam Creek, near Sharpsburg, Maryland.
On September 14, 1862, General Lee was seriously planning for a retreat from Maryland and move back into Virginia. He considered this because the force he sent to capture Harper's Ferry had yet to complete that order. Also, the Army of the Potomac had inflicted severe casualties on the Confederates at Turner's Gap on South Mountain.
Jefferson Davis ordered General Robert E. Lee to launch an offensive into Maryland in the summer of 1862, specifically in late August. This directive was part of Lee's broader strategy during the Civil War, culminating in the Battle of Antietam on September 17, 1862. The invasion aimed to shift the war into Union territory and gain support for the Confederate cause.
General Robert E. Lee instructed General James Longstreet to seize the Maryland towns of Boonsboro and Hagerstown. Lee's goal was to use both towns as a base of operations in Maryland.
The Battle of Boonsboro (South Mountain) was fought in Washington and Frederick counties in Maryland September 14, 1862. It was a part of Lee's Maryland Campaign which culminated in the bloody Battle of Antietam (Sharpsburg) September 17 1862. See the following link for more about the battle.
When General Robert E. Lee began his 1862 raid into Maryland, he failed to check on the Union garrison at Harper's Ferry. Because of this, his operations in Maryland were delayed until a portion of his army was sent to capture the federal arsenal.