In the summer of 1864 the Confederacy was in dire straits. Nearly a year after their crushing defeat at Gettysburg the Army of Northern Virginia was now being besieged by Grant around none other than the Confederate capital at Richmond. Lee was in a desperate position, and desperate times called for desperate measures.
The seeming last hope for the Confederacy would be pinned on Jubal Early. In June of that year Lee would send him on a special mission to clear Union forces out of the Valley and, if possible, threaten Washington. Lee hoped that at the very least this would take pressure off his men around Petersburg, but little did he know he would be handed something far more monumental than that...
The first warnings of the Confederate advance eastwards across Maryland would come not from a Union force, but from agents working for the Baltimore and Ohio Railroad. On June 29 workers from the railroad would report the presence of Confederate troops near Harpers Ferry to John Garrett, the president of the B&O. A strong Unionist, Garrett would forward these alarming reports to Major General Lew Wallace, commander of the Union Middle Department, on July 3, and alarming they were - a force of around 20, 000 enemy troops was said to be advancing down the B&O towards Catoctin Mountain and points east. Wallace’s hour had come.
In the summer of 1864 the men under Wallace’s command were still more or less untried. The vast majority of them were Hundred Days men, volunteers who had agreed to join the army for a period of a hundred days in a broader effort to free up veteran units for service at the front. At Frederick the Union presence was small, almost nonexistent. The first men to arrive near the town would be men from the Potomac Home Brigade under the command of Brigadier General Erastus Tyler. These men had been sent ahead of the main Union force on Wallace’s own personal accord (for he had received no orders from his superiors to move out), and soon took up defensive positions around Monocacy Junction, a couple of miles east of Frederick.
Monocacy Junction was an important position for the Union forces to hold. Not only was it an important railroad junction between the main branch of the B&O and a side branch into the town of Frederick, it was also located near two important east-west roads that could, if they fell into Rebel hands, prove easy routes of invasion towards Baltimore or Washington. The terrain at Monocacy was also suited for defense. To the west was a wide plain in the fertile Frederick Valley that slopped down to the Monocacy River. On the east side of the river were a series of limestone banks and bluffs that provided a natural defensive high ground to watch over the strategically important B&O bridge and the Georgetown Pike. Further to the east were a series of low, woody ridges upheld by resistant siltstone and slate that could also prove to be important obstacles to advancing Confederate troops. To make matters better was the presence of two blockhouses (one near the junction and another near the bluff where the B&O crossed the river), which were quickly occupied by Union troops.
At this point, however, Wallace was still unsure whether Baltimore or Washington was Early’s target, and occupying this crucial juncture would be the best position to not only delay the Confederate advance, but also gauge what Early’s target was. In other words, the Monocacy Junction would be an ideal location for Wallace to position his men, that is, except for one thing; it left the town of Frederick wide open for Confederate capture and gave him no fallback position should his men be overrun.
Over the course of July 3-7 Wallace would bring in more reinforcements from his command out of Baltimore, including the 8th Illinois Cavalry Regiment under the command of Lieutenant Colonel David Clendenin and the 11th Maryland Regiment, a light artillery unit. All in all the Union forces amounted to only around 1, 000 or so men, far less than the 20, 000 Confederates rapidly approaching from the west. To help augment his strength, Wallace would wire General Halleck for reinforcements, but these would prove too little too late. Before the first men from the VI Corps could arrive, Wallace’s force would be decimated at the Battle of Frederick.
The first clash between Early and Wallace would be on July 7. In an odd move, Wallace had sent his men away from the positions they had assumed near the junction to ones further west, outside of the town of Frederick, in an attempt to skirmish with the Confederates and delay their advance eastwards. Considering that Rickett’s division of the VI Corps was said to be on it’s way by rail it was perhaps more forgivable as, if the Confederates had rushed to the junction and overran the Union forces, there would have been no fallback option for Wallace's men afterwards, leaving the pathway open to Baltimore or Washington. By stationing his men at Frederick in a delaying action, he could hopefully buy time to allow Ricketts' division to arrive at Monocacy Junction and also keep the better positions at Monocacy as a much needed backup.
But this fact wouldn’t save Wallace’s men that fateful day. The terrain west of Frederick was much more evenly matched. West of Frederick but east of Catoctin Mountain the land gradually sloped up out of the limestone lowlands the town was situated in and onto higher standing shale and siltstone plains. In places these plains were dotted with low ridges of more resistant igneous rock, giving a slight advantage in topography to whoever commanded them. Although the Union men did take up positions around some of these hills, their small smize meant that when the brunt of the Confederate attack came on the 7th they were easily overwhelmed, hastily retreating to Frederick after just a brief skirmish. Here, after a spirited fight, Confederate cavalry would succeed in beating back the Illinoisans, and driving around Wallace’s tiny force, surrounding them and cutting the Union troops to pieces. Wallace himself would be pinned in a store in downtown Frederick with a mortally wounded Tyler as the remnants of the Union force, men from the 1st and 3rd Maryland Infantry Regiments, were quickly ground to a pulp by Virginians under Breckinridge’s command. Shortly afterwards Wallace would be forced to surrender(*).
With Wallace’s destruction on the 7th, word to outside Union forces of Early's advance was extremely slow to move. Halleck would stop receiving requests for more men and, as a result, confided to himself that Ricketts’ division was enough to satisfy Wallace and that no further action was needed. As for Ricketts, his 3, 000 men would meet a similar fate to Wallace’s. Without receiving word from Wallace on the status of the Monocacy Junction, his men would fall into a Confederate ambush, their entire force getting decimated by a combination of Confederate artillery and cavalry, which rode around and behind the Union troops, surrounding them and chopping them up piecemeal as they had their compatriots just a few hours earlier. Ricketts himself would be killed in the attack, sowing further confusion amongst his men as the survivors frantically attempted to flee in every which way. Total Union casualties for the two battles exceeded two thousand men, total Confederate losses numbered only around 500.
The fighting and the mid summer heat had taken their toll on the Confederate troops, however. Despite his outstanding achievement, Early would order his men to camp on the grounds just east of the Monocacy River on the night of the 7th-8th, delaying his march by a full day. But after their brief respite the Georgetown Pike lay completely open. There were no more Union troops standing between Early and the nation’s capital of Washington DC.
(*) This is the POD. IOTL Wallace actually did position his men west of Frederick, however after a brief skirmish he had them withdrawn to prevent their being surrounded and destroyed. ITTL Wallace is not able to do so, allowing Early to defeat the Northerners piecemeal.