General Wallace gathered up his scattered troops and prepared to meet the enemy at Monocacy. He was not well matched to meet them, but strongly resisted them long enough to enable Grant to reinforce Washington, and, strategically speaking, Wallace's fight saved Washington.
Appleton's Encyclopedia, page 130, under army operations 1864, says:
"Meantime the enemy after tearing up some railroad from Frederick to Baltimore, sent their main body south of it and detached a cavalry force towards the Northern Central Railroad from Harrisburg, Pa., to Baltimore. This Cavalry expedition overran Maryland, 25 miles of the Northern Central Railroad was destroyed, and on Monday the 11th (July), a force appeared on the Baltimore, Wilmington & Phila. Road and captured and set on fire the trains at Magnolia station, seventeen miles south of Havre de Grace.
In one train Major General Franklin was captured but afterwards made his escape. Some damage was done to the track and Gunpowder Bridge was partially burned. The Cavalry heavily loaded with plunder came within six miles of Baltimore, then turning southward they joined the force near Washington which had been sent in that direction to guard against surprise; part of it halted before Fort Stevens on 17th street."
I remained in Baltimore until July 14th, when I started out to scout the country east and north of the city.
Headquarters, Middle Department,
8th Army Corps.
Baltimore, July 14, 1864.
Pass H. B. Smith and George W. Thompson on Department business out and in Picket Lines at all hours.
By command Major General Wallace.
John Woolley,
Lt. Col. & Pro. Marshal.
General Wallace had been compelled (by Lee's invasion) to take away to Monocacy nearly all of his troops, and so we had to appeal to the citizens for the defence of the city. All loyal citizens were appealed to and they responded nobly; they made, however, a motley army, but patriotic to the core, they vigorously performed their duty.