Despite this criticism, the following day, August 30, was one of Longstreet's finest performances of the war. After his attacks on the 29th, Pope came to believe with little evidence that Jackson was in retreat. He ordered a reluctant Porter to move his corps forward in pursuit, and they collided with Jackson's men and suffered heavy casualties. The attack exposed the Union left flank, and Longstreet took advantage of this by launching a massive assault on the Union flank with over 25,000 men. For over four hours they "pounded like a giant hammer" with Longstreet actively directing artillery fire and sending brigades into the fray. Longstreet and Lee were together during the assault and both of them came under Union artillery fire. Although the Union troops put up a furious defense, Pope's army was forced to retreat in a manner similar to the embarrassing Union defeat at First Bull Run, fought on roughly the same battleground. Longstreet gave all of the credit for the victory to Lee, describing the campaign as "clever and brilliant". It established a strategic model he believed to be ideal—the use of defensive tactics within a strategic offensive. On September 1, Jackson's corps moved to cut off the Union retreat at the Battle of Chantilly. Longstreet's men remained on the field in order to fool Pope into thinking that Lee's entire army was still on his front.