On July 21, 1861, just days before the end of their 90-day term of service, the 5th Massachusetts took part in the First Battle of Bull Run, the first major engagement of Civil War. Before their departure from Alexandria, Major General Irvin McDowell, commanding the Union Army of Northeastern Virginia, addressed the 5th Massachusetts and, in light of their term of service being nearly at an end, offered them the option of foregoing the campaign. The 5th Massachusetts voted unanimously to go with McDowell's army. Assigned to the First Brigade (Franklin's), Third Division (Heintzelman's) of the Army of Northeastern Virginia, the 5th Massachusetts was among those units sent to probe the Confederate right flank on July 18 resulting in the Battle of Blackburn's Ford. When this maneuver failed, McDowell opted to send a large portion of his forces on a wide flanking maneuver across Sudley Springs Ford, hoping to get around the Confederate left flank. On July 21, precisely three months after they departed Boston, the 5th Massachusetts crossed Sudley Springs Ford and participated in pitched combat on Henry House Hill.