After it was determined that an advance north was inevitable, Longstreet dispatched the scout Henry Thomas Harrison, whom he had met during the Suffolk Campaign, to gather information. He paid Harrison in gold and told him that he "did not care to see him till he could bring information of importance". In the initial movements of the campaign, Longstreet's corps followed Ewell's through the Shenandoah Valley. Harrison reported to Longstreet on the evening of June 28, and was instrumental in warning the Confederates that the Army of the Potomac was advancing north to meet them more quickly than they had anticipated, and were already massing around Frederick, Maryland. Lee was initially skeptical, but the report prompted him to order the immediate concentration of his army north of Frederick near Gettysburg, Pennsylvania. Harrison also brought news that Joseph Hooker had been replaced as commander of the Army of the Potomac by George Meade.