At the Second Battle of Bull Run, McLean's brigade was placed at the extreme left of the Union line on Chinn Ridge. McDowell and Pope had placed him there to slow down a Confederate flank attack, which could have potentially overrun the rest of the Army of Virginia. McLean's troops took many casualties, but they were able to hold off the Confederates for a half-hour, which allowed Pope to establish a second line of defense further north of Chinn Ridge. This line of defense allowed the Army of Virginia to withdraw from the field without being destroyed.