A more simple explanation than I've read before.

"CHAPTER XII Stuart And The Gettysburg Campaign After Chancellorsville, the armies resumed their positions on the Rappahannock. A brilliant but barren victory had been won, and the pickets on the opposite banks of the river again began to trade in coffee and tobacco. With the years of hardship and danger, war had not lost all of its romance, and the soldiers observed in their intercourse the courtesies of combatants as strictly as did the Crusaders.

General Lee now determined to cross the Potomac and make a strategic offensive. His main object was really to create a diversion and conduct a great foraging expedition into Pennsylvania for the relief of Virginia and his fasting army — the South was almost exhausted. The movement would temporarily draw the enemy from Virginia, but he did not hope to dictate a peace north of the Potomac, nor could he have expected to maintain his army there without a line of communication and base of supply.

When Lee crossed the Potomac, he had no objective point. His army was now organized with three corps, under Longstreet, Ewell, and A. P. Hill — Stonewall Jackson had crossed the Great River. Stuart was his Chief of Cavalry.

Early in June the movement that terminated in the unexpected encounter at Gettysburg began from Fredericksburg up the river. Previously the cavalry corps had been sent in advance to Culpeper County to prevent the enemy's cavalry from crossing the Rappahannock and to get the benefit of the grazing ground. Lee followed with Longstreet and Ewell. A. P. Hill's corps was left behind to amuse Hooker. Lee wanted to conceal his march so that he could cross the Blue Ridge and surprise Milroy in the Shenandoah Valley. Hooker's man in the balloon discovered that some camp grounds had been abandoned, so a reconnaissance was ordered to find out what it meant. But the force met with such resistance that Hooker concluded that Lee's whole army was there.

To relieve the Administration of anxiety about invasion, Hooker telegraphed to Washington what the reconnoitring force reported — just what Lee wanted him to do. The impression was confirmed by pretended deserters, who said they belonged to reinforcements that had just come to Lee. Deception is the ethics of war."

____________________
David Upton