On August 30, 1862, as Union forces prepared to attack General Jackson's Confederate line along the unfinished railroad north of the field, Union General John F. Reynolds personally reconnoitered beyond his skirmishers along the Groveton Road and drew fire from a Confederate skirmish line in the open ground that was covering General James Longstreet's massing attack against the Union left flank. Reynolds then shifted his three brigades of Pennsylvania Reserves toward the Chinn Farm to better guard the Union left and rear, but the move opened a dangerous gap south of the Warrenton Turnpike. To support Lt. Charles Hazlett's Battery D, 5th U.S. Artillery at Groveton, Colonel G. K. Warren placed his small brigade of New York Zouaves in the position Reynolds' entire division had occupied. At 4:00 p.m. Longstreet's Confederates advanced, broke through Warren's two regiments east of the Groveton Road, and gained Chinn Ridge before encountering significant resistance. The attack was not checked until the lead brigades reached the Sudley Road 1-1/2 miles to the east, where Union General John Pope had hastily formed a strong defensive position on Henry Hill. After dark the Union army retreated across Bull Run, ending the battle. In Civil War armies, the regiment was the basic organizational unit, usually a numbered state volunteer formation with a headquarters staff and ten companies lettered A through K, without J; companies initially mustered with 100 men and officers, though losses, disease, and desertion often reduced their strength. Up to five regiments could be grouped into a brigade, and two to four brigades formed a division.
