Both sides had roughly corps-sized units in the battle.

Total casualties were about 2,400.

After marching north through the Shenandoah Valley from Lynchburg, Early's army side-stepped the Federal garrison at Harpers Ferry and crossed the Potomac River at Shepherdstown into Maryland on July 5-6. On July 9, 1864, a makeshift Union force under Maj. Gen. Lew Wallace attempted to arrest Early's invading Confederate divisions along the Monocacy River, just east of Frederick.

Wallace had mainly Home Guards and militia, but was joined by Ricketts's Division of the VI Corps that had been rushed from the Petersburg lines. He had two areas to defend: the coastal Maryland cities which had strong Confederate sympathies, and Washington DC - a political target if there ever was one. He knew he wasn't likely to beat Early's veterans, nor could he pull back and defend one target, because Early would simply move on the other. So he bravely moved west and made sure he would delay (if not defeat) Early.

He took position behind the Monocacy River, east of Frederick (where the Confederates had just levied $200,000 under threat of burning the city). The Union line brought the Confederate advance guard under enough fire that it had to deploy; then a second division had to deploy and turn Wallace's flank. The fighting was heavy, and Wallace had to pull back or face annihilation. Early didn't pursue, though he might have been able to destroy Wallace's force. Instead he kept his eye on the main prize, and moved his men own the Washington road.

Hearing of Early's incursion into Maryland, Grant embarked the rest of the VI Corps on transports at City Point, sending it with all dispatch to Washington. Wallace's defeat at Monocacy bought time for the veterans to arrive to bolster the defenses of Washington. Monocacy was called the "Battle that Saved Washington."

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Selected sources:
American Battlefield Protection Program, Heritage Preservation Services, National Park Service.