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Commanded by Brigadier General Samuel P. Heintzelman
The Battle of Bull Run was fought on July 21.
| Unattached | ||
| 4th United States Cavalry, Cos. A&E | Captain James B. McIntyre | |
| First Brigade | Colonel W. B. Franklin | |
| 5th Massachusetts Infantry Regiment | Colonel S. C. Lawrence (w 7/21) | from Dept. Washington 7/16 to Boston to muster out 7/29 |
| 11th Massachusetts Infantry Regiment | Colonel G. Clark | |
| 1st Minnesota Infantry Regiment | Colonel W. A. Gorman | from Dept. Washington 7/3 |
| 4th Pennsylvania Infantry Regiment | Colonel John Hartranft | refused to extend service on eve of battle and mustered out 7/20 |
| 5th Pennsylvania Infantry Regiment | Colonel R. P. McDowell | remained at Alexandria during battle and mustered out at Harrisburg 7/25 |
| 1st United States Artillery, Battery I | Captain J. B. Ricketts (w&c 7/21) Lieutenant E. Kirby |
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| Second Brigade | Col. O. B. Wilcox (w&c 7/21) Colonel J. H. Hobart Ward |
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| 11th New York Infantry Regiment | Colonel Noah L. Farnham (mw 7/21) Lt. Colonel John A. Cregier |
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| 38th New York Infantry Regiment | Colonel J. H. Ward (^ 7/21) Lt. Colonel Addison Farnsworth |
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| 1st Michigan Infantry Regiment | Major A. F. Bidwell | |
| 4th Michigan Infantry Regiment | Colonel D. A. Woodbury | |
| 2nd United States Artillery, Battery D | Captain Richard Arnold | |
| Third Brigade | Col. O. O. Howard | |
| 3rd Maine Infantry Regiment | Major H. G. Staples | |
| 4th Maine Infantry Regiment | Colonel H. G. Berry | |
| 5th Maine Infantry Regiment | Colonel M. H. Dunnell | |
| 2nd Vermont Infantry Regiment | Colonel H. Whiting |