Also at 6 p.m., on the Union left flank, Burnside advanced along the Fredericksburg Road. Both he and Grant were unaware that when Lee had moved units to the Po, he had left only Cadmus Wilcox's Confederate division to defend that avenue of approach and that there was a large gap between Wilcox and Ewell. (This lack of information was a tangible consequence of the decision to send all of Sheridan's cavalry away from the battlefield.) As Burnside began to get resistance from Wilcox, he timidly stopped and entrenched. That evening Grant decided that Burnside was too isolated from the rest of the line and ordered him to pull back behind the Ni and move to join his lines with Wright's. Grant wrote about this significant lost opportunity in his "Personal Memoirs":