The battle was part of Early's raid through the Shenandoah Valley and into Maryland in an attempt to divert Union forces from their siege of Gen. Robert E. Lee's army at Petersburg, Virginia.

In May and June, as part of the Valley Campaigns of 1864, Union General-in-Chief Lt. Gen. Ulysses S. Grant planned a coordinated movement of troops against Confederates in Virginia.

[5] Grant reacted to Early's raids at Martinsburg and Harpers Ferry by dispatching two brigades, about 5,000 men, from the 3rd Division of the VI Corps, under Brig.

[7] Although Wallace had experience as a battlefield commander, having been the Union Army's youngest major general at the time of his promotion, his military career faltered after he was blamed for the high casualties taken on the first day at the Battle of Shiloh.

[8] In the meantime, agents of the Baltimore and Ohio Railroad at Cumberland, Maryland, and Harpers Ferry reported signs of Early's advance on June 29.

John W. Garrett, the president of the railroad and a Union supporter, passed this intelligence, and subsequent reports reached Wallace on July 3.

The bridge was important to rail connections at Harpers Ferry and points farther west, so Wallace agreed to bring a force to the Monocacy to protect it.

After Garrett's agents reported the Confederate attack at Harpers Ferry, Wallace relayed the information to Gen. Henry Halleck, then ordered his brigades at Baltimore to be ready to move.

[3][14] Following the skirmishing on July 7 and 8, when Confederate cavalry drove Union units from Frederick, Early demanded, and received, $200,000 ransom from the city to save it from destruction.

[15] Wallace's prospects improved when word arrived that the first contingent of VI Corps troops, commanded by Ricketts, had reached Baltimore.

[3] Although they were originally ordered to travel by rail to Harpers Ferry, Ricketts and about 3,300 of his men arrived at Monocacy Junction, where Wallace stopped them on the evening of July 8.

The higher elevation of the river's east bank formed a natural breastwork for some of the men, while Tyler's brigade occupied the two block-houses and trenches his soldiers had dug near the bridges.

McCausland crossed the Monocacy below the McKinney-Worthington ford, about a mile downstream from the Georgetown Pike bridge, and attacked Wallace's left flank.

[17] Due to the rolling terrain, McCausland's men did not notice that four regiments of Ricketts's veteran troops had taken a position along a fence separating the Worthington and Thomas farms.

Pressure from Ramseur's attack on the Union center and Confederate artillery fire from across the river kept Wallace from reinforcing Ricketts's men.

[3] By late afternoon on July 9, following this northernmost Confederate victory of the war, the Federals were retreating toward Baltimore, leaving behind more than 1,294 dead, wounded, or captured.

[1][30] Early's army had won the field at Monocacy, but at the cost of an estimated 700 to 1,000 men killed or wounded[1][28][31] and the loss of a day's march.

One critical objective had been accomplished: the Union troops at Monocacy Junction had delayed Early’s advance on Washington by a full day.

[39] General Early wrote in a report of the 1864 campaign: Some of the Northern papers stated that, between Saturday and Monday, I could have entered the city; but on Saturday I was fighting at Monocacy, thirty-five miles from Washington, a force which I could not leave in my rear; and after disposing of that force and moving as rapidly as it was possible for me to move, I did not arrive in front of the fortifications until after noon on Monday, and then my troops were exhausted ...[40]Grant assessed Wallace's delaying tactics at Monocacy in his memoirs: If Early had been but one day earlier, he might have entered the capital before the arrival of the reinforcements I had sent .... General Wallace contributed on this occasion by the defeat of the troops under him, a greater benefit to the cause than often falls to the lot of a commander of an equal force to render by means of a victory.

Five monuments were erected at the site, including tributes to Union troops from New Jersey, Vermont, and Pennsylvania, as well as to the Confederate force who fought in the battle.