August 30 proved to be one of the darkest days of the war for Union arms when the balance of the Confederate forces under Major General James Longstreet descended upon the vulnerable Federal left flank. The Yankees were routed, and the army was saved from virtual destruction only by a heroic rear guard action. Pope gathered his demoralized army around him at Centerville and waited for the next blow to land, and on the very next day Jackson obligingly delivered the punch at Chantilly, Virginia. In a driving rainstorm, the blue and butternut forces surged back and forth across mud-soaked fields until darkness put an end to the fighting. Although considered a draw, Pope and his men retreated closer to the fortifications surrounding Washington, D.C., where they struggled to regroup, much like a boxer cowering under his gloves with his back against the ropes.

While the Union army was in the process of being mauled at the Second Battle of Manassas, the remains of the 12th huddled near McClellan's headquarters outside Alexandria. Deprived of their base but temporarily free from any responsibility, the Pennsylvanians lived off odds and ends of rations and forage scrounged from the garrison troops around the city, while using the time to calm their jangled nerves. They would only be allowed a short period to convalesce.

Either because suspicions about the abilities of General Pope led to an expectation of defeat or out of newly gained respect for General Lee, the Union high command began to fret that the war might soon be headed into Northern territory. On August 30, while the conflict raged along the Bull Run, three hundred of the Hussars under the command of Major Congdon were dispatched on a reconnaissance mission along the banks of the Potomac River as far west as Edward's Ferry. The next day a squadron, perhaps all of the 12th not sent on the scout, was detailed to Fort Lyon, probably to act as pickets for the garrison of nearly a thousand New Yorkers stationed there.

Congdon's detachment was only permitted a brief sojourn at Edward's Ferry. According to the major, on September 2 the Regiment was sent west on a 52-mile trek to a stop on the Baltimore & Ohio Railroad known as Sir John's Run, which was located a few miles west of Hancock, Maryland. As a reward for their marathon ride, the weary and saddle-sore troopers, and their even wearier mounts, were then permitted to remain in bivouac for several days along the banks of the tributary to the Potomac for which the station had been named. In a letter home, Congdon groused that the Regiment's ordeal had been endured for nothing, but it was only as a result of blind chance that his assessment was justified. The day after the letter was written, on September 4, 1862, Lee and his army began to cross the Potomac into Federal territory at White's Ford which was the same spot, several miles west of Edward's Ferry, that Congdon and his men had passed only two days before. Although the main body of the Regiment just missed the opportunity to obtain critical intelligence about Lee's invasion of the North, another Hussar was able to provide some useful information to headquarters.

On September 3, Major Titus straggled into the Union camp at Upton's Hill, Virginia, having been paroled from his capture near Manassas Junction a week earlier. Despite his captivity, Titus paid close attention to events unfolding around him and was thus able to provide a detailed recitation of the Confederate troop movements he had observed. At Gum Springs, Virginia, Titus watched as Butternut infantry shuffled past him from 3:00 a.m. until after dark in the direction, according to one loose-lipped Johnnie, of Harpers Ferry. After his parole was taken, the major was permitted to return to his own lines. When he reached the Centerville area, Titus was diverted away from the Little River Pike so as not to be able to observe the troops moving on that road, but he was never the less able to report that he spied a column of marching troops and some artillery. Although McClellan, the newly reappointed commander of the Virginia theater, did not know exactly where his adversaries were headed, he did know that he had better soon find out.

Determining the location and strength of the enemy, in an era before satellites or aircraft, was a job for which cavalry was ideally suited and usually employed. Brigadier General Alfred Pleasonton, commander of the five brigade, twelve regiment, cavalry division of the Army of the Potomac, was handed that crucial assignment and on September 4 he began trying to ferret out the Rebels' exact location and, if possible, their intentions. The next several days were occupied by redeploying the troopers of his division, first over the Potomac River by way of the Aqueduct Bridge, then north to Tennallytown, D.C., and finally westward to reconnoiter all of the fords over the Potomac as far west as Muddy Run. Despite the exertions of the Yankee troopers, the Rebels' location remained a mystery because the bivouac chosen for the night of September 6 was still far to the east of the enemy.

The next day Pleasonton dispersed his horsemen westward in a pattern that resembled a hand placed on a map of Maryland, with one finger pointed toward Poolesville, Maryland, and the Potomac fords and the northernmost finger towards Brookville, Maryland. In the center of the pattern, the newly constituted Fourth Brigade, which consisted of the 12th and the 1st New York Cavalry, moved through Middlebrook on its way toward the occupation of Clarksburg. At the same time that the Union troopers were sniffing about central Maryland, their quarry was slowly spreading out from White's Ferry in a mostly northward direction behind the Monocacy River towards Frederick, Maryland. Collisions were inevitable and as a result, from September 7 until September 11 the Union cavalry became engaged in a number of running skirmishes all along the periphery of the Confederate army. With the dimensions of the crisis sufficiently developed, McClellan began to shift his army west in hopes of expelling the gray invaders from Union soil.

As the bulk of the Federal army moved out from Washington, the Confederates began a contraction behind the natural defenses offered by the parallel ridge lines to their west: first the Catoctin Mountains, and then the ridge known as South Mountain. Mysteriously, while all of this movement was taking place, Major Congdon's detachment somehow managed to rejoin the bulk of Pleasonton's division near Frederick, Maryland, in time to participate in the liberation of that town on September 12. How they managed to pass from Sir John's Run to Frederick while the entire Confederate army filled the area between those two points has apparently been lost to history. Perhaps they traversed the chord by traveling south through northern Virginia or, more likely, they made a loop to the north through Emmitsburg, Maryland. In either case, the Pennsylvanians' return journey must have been at least as arduous as their departure a week earlier.

In partial compensation for the rigors of their trek the members of the 12th, as part of Pleasonton's division, received heros' welcomes when they paraded down Urbana Road and into Frederick at 5:00 p.m. on September 12. The scene greatly impressed the recipients of the adulation. "The Federal army had not before seen such a reception as was here given it. In every doorway and at every window were women and young girls waving the Union flag and in every way manifesting the greatest joy. [We] were getting in the middle of all this flag waving, singing, shouting, joy,and,caresses.

The Hussars and their comrades were dragged back to reality the next morning when orders arrived from headquarters dispatching their Fourth Brigade to conduct a reconnaissance toward Gettysburg in order to determine if Lee had his sights set on the Keystone State. As the dusty blue column moved northward, they were overtaken from the rear by two Confederate cavalry regiments doing some scouting of their own. A brief spat ensued. Both sides claimed a few prisoners but apparently no casualties were suffered by either. The Confederates soon broke off the engagement having located their adversaries, and the Federals continued toward Emmitsburg pursuant to their orders. The balance of September 13 was spent on the 25-mile ride to Emmitsburg, which was on the way to Gettysburg. Curiously, the former town, unlike Frederick to its south, harbored strong secessionist sentiment and feted some of the first Federals to arrive there in the mistaken belief that the dirty and faded new arrivals were Rebels. Aggravated by their mistake, the disappointed citizens nevertheless allowed the Federals a peaceful night, after which the Fourth Brigade resumed its mission in the morning, enjoying in the process a pleasant ride and the generosity of local farmers who bolstered the thirsty troopers with fresh fruit, cold milk, and words of encouragement. In contrast to that peaceful passage, a minor panic occurred when the column reached the outskirts of Gettysburg in the early afternoon of Sunday, September 14. Several congregations were sent scurrying home in the middle of their devotions out of fear that the town was being invaded by Confederates. When the truth was finally discovered the citizens welcomed the weary horse soldiers and invited them to share in evening prayers.

Those in need of repairs to horses or equipments used the quiet Monday morning to visit the local blacksmith and tack shops. Then, having established that the enemy had not breached the Mason-Dixon line, the Union cavalry commenced its return journey in the afternoon. The column reached Emmitsburg late that day and at least the 1st New York Cavalry arrived in Frederick on September 16.

At some point prior to reaching Frederick, the 12th must have received orders to send small detachments to scout the local vicinity and for the majority of the Regiment to join the main body of the army gathered around Sharpsburg, Maryland. Two squadrons under Captains Adam Hartman (Company G) and William Linton (Company M) cantered off in the direction of Hagerstown, Maryland, for a look-see. Late in the day, their detachment made contact with the enemy about two miles west of Boonsboro. Shots were exchanged and the Hussars bagged a few Butternut prisoners.'4 After the skirmish, Hartman's and Linton's men probably rejoined the rest of the Regiment, which was by then bivouacked in the rear of the Army of the Potomac. All of the men in blue squatted around snapping and flickering campfires that dark night seeking to drive away the chill creeping into their bones from the September dampness, and into their souls from mortal fear about fate and the battle which was surely coming.

While the Fourth Cavalry Brigade had been off on its jaunt into Pennsylvania, the Army of the Potomac closed in on the Confederate invaders. Thanks to the chance discovery of Lee's battle plan wrapped around some dropped cigars, McClellan learned that the Southern forces were divided between those under Jackson, which had been sent to capture Harpers Ferry, and the balance of the Army of Northern Virginia which waited behind South Mountain. On September 14, the Union army spent almost the entire day forcing the passes through that mountain range. After the passes were cleared, the Federals spent the next two days gathering themselves for an assault on Lee's army, which had assumed a defensive position behind the Antietam Creek outside of the town of Sharpsburg, Maryland.