After the fighting at Cold Harbor the the 138th PA was ordered to proceed to City Point on June 6th. From there they would move to the site where they had first become a true fighting unit Harpers Ferry. A Confederated detachement under General Jubel Early was advancing north and threatening Washington. 6th Corps which included the 138th was being sent to stop him. On July 8th the unit arrived at City point on the Monacacy River. Garrison troops were already trying to stave off the Confederate advance. The true battle began the next day July 9th. Most of the 6th Corp was at Washington at this time leaving the 138th to wait and hold out till they arrived. The Union troops were outnumbered 3 to 1 but held stubbornly. After a prolonged fight the Union line began to give way and the Federal units including the 138th PA retreated. While the battle was a defeat it bought enough time for the rest of 6th Corps to reach Maryland and force Jubel Early to retreat back into the Shenendoah Valley. The 138th PA Infantry would suffer 68 casualties.After the battle Uylles S. Grant decided that the valley should be wiped clean of both Confederates and their food supplies. Grant created the Middle Military Division with General Sheridan. The 138th PA infantry would be part of this new division.