The last of Lee's infantry was crossing the river by early afternoon on September 19. Lee then sent General J.E.B. Stuart's cavalry to a second crossing near Williamsport, Maryland. Just as the last of the Confederates were wading ashore, Union General Alfred Pleasonton arrived with a scouting force of Union cavalrymen. He had orders from Major general George B. McClellan not to follow the retreating rebels into Virginia "unless you see a splendid opportunity to inflict great damage upon the enemy without loss to yourself." Seeing the Confederates on the South side covered by artillery on the bluffs above, he did not immediately cross the river. Instead he set up 70 cannons on the north side of the Potomac. He then ordered the cannons to open fire on the Confederates. The Confederate artillery returned fire as best they could. That same morning Union general Fitz John Porter's V Corps had arrived. He had orders from McClellan to support Pleasonton and attack the Confederates as they retreated. As night approached, Porter's 500 or so Union soldiers began crossing the Potomac. The Union troops attacked Lee's rearguard and proceeded to capture several of Pendleton's cannon. With his artillery overpowered Pendleton panicked and left his post as commander of the Confederate forces to look for reinforcements. Shortly after midnight Pendleton found Lee. He told his commander he had lost all of his cannon and Federal troops were on the South shore. In fact, the Union forces had only captured five of his cannons. In his absence, the Confederate artillerymen had moved the remaining cannon back to safety.