On the 27th of February, 1862, the regiment was ordered to Washington, and, upon its arrival, went into camp near Bladensburg Cemetery. Company and regimental drill was practiced under the strictest discipline. Here the altered flint-lock muskets furnished by the State, were exchanged for the Belgian Rifles. On the 25th of March, the regiment was ordered to report to Brigadier Keim, of Casey's Division; but, on the following day, the order was countermanded, and, on the 29th, it was ordered to proceed to Harper's Ferry, and report to Colonel Miles. Upon his arrival, Colonel Campbell was directed to make the following disposition of his force along the line the Baltimore and Ohio Railroad: Company G, Captain F.B. Long, at Back Creek Bridge, eight miles west of Martinsburg; company F, Captain G.W. B. Davis, at Sleepy Creek Bridge, nine miles further west; company D, Captain Thomas H. Lapsly, at Alpine Station, five miles beyond the Sleepy Creek post; company I, Captain William B. Bonacker, at Sir John's Run, six mile further west; company C, Captain E.D. Yutzy, at Great Cacapon Bridge, five miles west of sir John's Run; company H, Captain John O. Billheimer, at Rockwell's Run, six miles beyond; company E, Captain Patrick Graham, at No. 12 Water Station; Company B, Captain John H. Hite, at Paw Paw, three or four miles further on; company K, Captain Edmond R. Newhard, at Little Cacapon Bridge, and company A, Captain John P. Suter, at South Branch Bridge, sixteen miles east of Cumberland, and sixty-two miles west of Martinsburg. The distance to be guarded was fifty-six miles. Regimental headquarters were established at first at Great Cacapon, but subsequently at Sir John's Run.

The country through which that portion of the railroad runs, which the regiment was required to guard, was considered by the rebels as their own territory, and the majority of the population, in the vicinity, was rebel at heart. Numerous guerrilla bands, led by daring and reckless chieftains, roved the country, pillaging and burning the property of Union inhabitants, and watchful for and opportunity to burn the railroad bridges, cut the wires of the telegraph, and destroy the road. To guard this great thoroughfare, of vital importance to the government, to suppress guerrilla warfare, to afford protection to the harassed and helpless people, was the duty which the regiment was assigned to perform. Colonel Campbell at once assumed, as a cardinal principle, that the true way to deal with guerrillas was to assume the offensive, and hunt them, instead of waiting to let them hunt him. Almost daily, for some part of the line, squads were sent out to engage and capture these roving bands, led by such notorious partisans as Edwards, | White, | Imboden, | and M'Neil, and many were brought in. Some of these were sent to Harper's Ferry, others, less guilty, upon taking the oath of allegiance, were released, while the most notorious were sent to Camp Chase, Ohio.

On Sunday morning, the 25th of May, 1862, all the locomotives on the railroad west of Harper's Ferry, were hurried through to Cumberland, the engineers bringing the first intelligence of the retreat of Banks, and the approach of Stonewall Jackson to Martinsburg. At nine o'clock that night Colonel Campbell received the following dispatch from Colonel Miles: "Concentrate your regiment at South Branch. General Banks defeated and driven through Martinsburg. Expect an attack here hourly. Mean to fight."

Fortunately, Colonel Campbell had detained one engine, and this he immediately dispatched for company G, nearest to Martinsburg, and most exposed to the enemies advancing columns. It arrived not a moment too soon; for as the company moved away the rebel advance began to swarm in, and soon set fire to the deserted camp, and the railroad bridge which the company had been guarding. A train procured from Cumberland was sent out and the companies were all successively picked up and carried west to South Branch. Here the Potomac was spanned by a substantial iron bridge, the only one left standing by the rebels during their occupation of the road in the preceding summer. The telegraph wires were now severed and all communication with headquarters was cut off. Lieutenant Colonel M'Dermit was sent out with two companies on the 28th to reconnoiter, and on the 31st Colonel Campbell proceeded down the road with two companies as far as Back Creek, without meeting any opposition. The destruction of the bridge at this point prevented further progress, and he returned to camp. On the 1st of June he dispatched two companies under command of Major Linton to the Great Cacapon, for the protection of the bridge. In the meantime, Jackson had been driven from the valley, and on the following day the Colonel received the following order, by telegraph, from Colonel Miles: "Colonel Campbell and command will occupy the railroad as before." By nightfall all the companies were at their old posts.

The success of Jackson, and the consequent withdrawal from the road, had inspired his roving bands with new life, and they became more troublesome than ever, wandering up and down the country, pillaging, indiscriminately, from friend and foe. The several companies were kept constantly on the alert, and with an energy and enterprise rarely equaled, the territory was scoured, many of the squads penetrating the interior twenty and thirty miles, capturing and dispersing the guerrillas, restoring stolen property, and successfully protecting and preserving the road. From June 1st to September 10th, two hundred and thirteen guerrillas and two hundred and seventy-three horses, together with muskets, sabers, pistols and other military trappings, were captured.

The rebel army having defeated M'Clellan upon the Peninsula and Pope at Bull Run, was now advancing into Maryland on the Antietam campaign. On the 11th of September his advance guard reached Back Creek, where he surprised and captured some of the pickets of company G. Communication with Colonel Miles was again severed, and soon after Harper's Ferry was invested by Jackson, the post, garrison, and immense military stores falling into the hands of the enemy. Colonel Campbell now telegraphed to General Kelly, in command in West Virginia, for orders. Kelly declined to give any, but advised the withdrawal from the road This the Colonel decided not to follow, and clung to his position, which had now become perilous, his little band of nine hundred men, without artillery or cavalry, being the only Union forces, at that time, in the hostile territory of Virginia.

On the 12th, Colonel Campbell, taking a small detachment from company I, Captain Bonacker, at Sir John' Run, and another from company D, under Lieutenant Gageby, proceeded in open platform cars to Back Creek. There he was joined by Captain Long, with a small force, in all but sixty-six men. Proceeding cautiously towards North Mountain, and awaiting until the main body of the enemy had passed, Colonel Campbell made a bold and impetuous attack upon his rear guard. The rebels, ignorant of the numbers of the attacking force, were thrown into confusion, and precipitately fled. In this spirited sortie two of the enemy were left dead upon the field, seven were wounded, nineteen were taken prisoners, and thirty stands of arms and one caisson were captured. Leaving a small force, with Major Linton in command, at Back Creek, Colonel Campbell returned to headquarters, and immediately re-established telegraphic communication with the advance post. Two days later the enemy again advanced his pickets to the neighborhood of Back Creek. A division of his army was engaged in destroying the railroad, with headquarters at Martinsburg. Learning that the enemy's pickets had advanced, Colonel Campbell, with detachments from companies C, D, and I, hastened to the support of Major Linton. "From their stations," says a correspondent, "the enemy's pickets could see but one side of the Colonel's camp at the bridge, and struck with this fact, he conceived and executed a happy device. Leading in his detachments on the exposed side, he marched them over the hill, out of sight of the enemy, and again and again back to the road and through the camp, thus conveying to the rebel pickets the impression that the post had been strongly reinforced. That night three hollowed logs were mounted upon the wooden breast-works, and with the soldiers' gum blankets wrapped around them, presented the appearance of formidable siege guns. To complete the deception the men bored holes in the stumps with a large auger, and charging them freely with powder, set them off regularly morning and evening."

Skirmishing with the enemies pickets was kept up until the 21st, when company G, which was holding the bridge, was attacked by a considerable force of the enemy. Upon receipt of intelligence of its approach, Colonel Campbell moved his train, with his forces hastily collected, to the support of the menaced detachment; but before he arrived, Major Linton had been attacked with an overwhelming force, and was obliged to fall back, bringing off all his men, but losing his camp and garrison equipage. The enemy advanced and burned the railroad bridge, but hearing the whistle of the locomotive bringing up Colonel Campbell's force, and suspecting an ambuscade, and the arrival of reinforcements, hastily fell back. Colonel Campbell pushed out his pickets again to the bridge, where they arrived before the ruins had been extinguished.

After the battle of Antietam, General M'Clellan, unaware of the presence of any Union troops south of the Potomac, sent a cavalry force to picket the Maryland shore. Seeing soldiers in blue across the river, they regarded Colonel Campbell's men as rebels in disguise, and it was with difficulty that they could be undeceived. Upon the surrender of Miles, the brigade to which the regiment belonged, had disappeared. A report of its position to the General-in-Chief soon brought an order attaching it to General Franklin's command. At daylight on the 4th of October, the rebel General Imboden, with a force of infantry and cavalry, seven hundred strong, attacked company K, Captain Newhard, at Little Cacapon. The men were at roll-call when the enemy, under cover of a dense fog, rushed into their camp, and the first intimation that they had of his presence was a volley fired from their own rifle-pits. They immediately seized their arms, and attempted to drive out the hostile force; but the odds were too great, and they were forced to yield. Thirty-five of the company escaped; but Captain Newhard, and fifty of his men were captured. Seven of the company were wounded. The enemy had two killed and eight wounded. Moving rapidly to Paw Paw, where Captain Hite was stationed with company B, Imboden divided his brigade, and, with one column keeping the Captain amused in front, sent the other to the rear, and before the latter was aware of his situation, he was fast between the two. Bringing up two small guns within easy range of Hite's position, Imboden demanded his surrender. As resistance was sure to entail a useless slaughter, the company yielded. Learning the fate of two of his companies, Colonel Campbell quickly concentrated his force at Sir John's Run. Imboden approached within six miles, and after reconnoitering the position for a day, unwilling to attack, withdrew to Winchester, and the remaining companies of the Fifty-fourth were returned to their old posts.