1st Maryland Infantry , Potomac Home Brigade The 1st Maryland Infantry , Potomac Home Brigade was an infantry regiment that served in the Union Army during the American Civil War . Service . The 1st Maryland Infantry , Potomac Home Brigade was organized at Frederick , Maryland beginning August 15 , 1861 and mustered in on December 13 , 1861 for three years under the command of Colonel William P . Maulsby Sr . Companies A , B , D and I were recruited in Frederick County . Company C was recruited from Baltimore City . Companies E , F and H were recruited from Washington County . Company G comprised men from Baltimore , Carroll and Frederick Counties . Company K was from recruited Baltimore city and Frederick County . The regiment was attached to Banks ' Division , Army of the Potomac , to March 1862 . Unassigned , Banks ' V Corps , and Department of the Shenandoah to May 1862 . Railroad District , Middle Department , to July 1862 . Railroad District , VIII Corps , Middle Department , to September 1862 . Annapolis , Maryland , VIII Corps , to March 1863 . 1st Separate Brigade , VIII Corps , to June 1863 . Lockwood ' s Brigade , VIII Corps , to July 1863 . 2nd Brigade , 1st Division , XII Corps , Army of the Potomac , July 1863 . 2nd Brigade , Maryland Heights , Division West Virginia , to December 1863 . 2nd Brigade , 1st Division , West Virginia , to April 1864 . Reserve Division , Harpers Ferry , West Virginia , to January 1865 . 3rd Brigade , 3rd Division , West Virginia , to April 1865 . The 1st Maryland Infantry , Potomac Home Brigade mustered out of the service August through December 1864 . The regiment continued to recruit for veteran service , and its designation was changed to the 13th Maryland Infantry . Detailed service . Railroad guard duty until March 1862 . Advance on Winchester , Va . , March 7 – 12 . Strasburg March 27 . Guarding Baltimore & Ohio Railroad until May . Concentrated at Harpers Ferry May 24 , and action at Loudon Heights May 27 . Defense of Harpers Ferry May 28 – 30 . Guard Baltimore & Ohio Railroad until September . Action at Monocacy Aqueduct September 4 . Poolesville September 5 . Concentrated at Sandy Hook and march to Harpers Ferry . Siege of Harper ' s Ferry September 12 – 15 . Maryland Heights September 13 . Harpers Ferry September 14 – 15 . Surrendered September 15 and paroled September 16 . Sent to Annapolis , Md . , and when exchanged assigned to duty on the Potomac in southern Maryland to June 1863 . Martinsburg June 14 . At point Lookout June . Joined Lockwood ' s Brigade and march to Gettysburg , Pa . , June 25 - July 2 . Battle of Gettysburg July 2 – 3 . Pursuit of Lee July 5 – 24 . Guard duty on Baltimore Ohio Railroad in Maryland and Virginia until May 1864 . Operations against Early ' s invasion of Maryland June and July . Duffield Station June 29 . Battle of Monocacy July 9 . Moved from Monocacy to Harpers Ferry , West Virginia , and duty in that district until April 1865 . Gettysburg . First Day . Under command of Col . Maulsby , the regiment formed part of 2nd Brigade ( Brig . Gen . Henry H . Lockwood ) , 1st Division ( Brig . Gen . Alpheus S . Williams ) , XII Corps ( Maj . Gen . Henry W . Slocum ) at the Battle of Gettysburg . The regiment mustered 674 all ranks , making it the largest regiment in the Army of the Potomac . The regiment reached Gettysburg during the afternoon of July 1 , but was not engaged . Second Day . On the morning of July 2 , the regiment constructed breastwork s south of Culp ' s Hill at McAlister ' s Woods . The entire division was withdrawn later in the day and sent south to support a section of the Union line under attack by Longstreet ' s Corps . Returning to Culp ' s Hill that evening , the regiment discovered that its breastworks had been captured by Confederates of Maj . Gen . Edward " Allegheny " Johnson ' s division . Third Day . On July 3 , the regiment fought to hold ground against renewed attacks by Johnson ' s division . Bloody fighting continued until around 11 : 00 a . m . when Johnson finally broke off his attack . XII Corps commander Brig . Gen . Alpheus Williams wrote : " The wonder is that the rebels persisted so long in an attempt that the first half hour must have told them was useless . " Losses . The regiment mustered 674 all ranks at the beginning of the battle . It lost 104 killed , wounded , captured and missing . Casualties . The regiment lost a total of 131 men during service ; 3 officers and 42 enlisted men killed or mortally wounded , 1 officer and 85 enlisted men died of disease .