Sigel, who was in command of our forces there, retreated across
the Potomac at Shepherdtown; and General Weber, commanding at Harper's
Ferry, crossed the occupied Hagerstown, moving a strong column towards
Frederick City. General Wallace, with Rickett's division and his own
command, the latter mostly new and undisciplined troops, pushed out from
Baltimore with great promptness, and met the enemy in force on the
Monocacy, near the crossing of the railroad bridge. His force was not
sufficient to insure success, but he fought the enemy nevertheless, and
although it resulted in a defeat to our arms, yet it detained the enemy,
and thereby served to enable General Wright to reach Washington with two
division of the 6th corps, and the advance of the 19th corps, before
him. From Monocacy the enemy moved on Washington, his cavalry advance
reaching Rockville on the evening of the 10th. On the 12th a
reconnoissance was thrown out in front of Fort Stevens, to ascertain the
enemy's position and force. A severe skirmish ensued, in which we lost
about two hundred and eighty in killed and wounded. The enemy's loss
was probably greater. He commenced retreating during the night.
Learning the exact condition of affairs at Washington, I requested by
telegraph, at forty-five minutes past eleven P.M., on the 12th, the
assignment of Major-General H. G. Wright to the command of all the
troops that could be made available to operate in the field against the
enemy, and directed that he should get outside of the trenches with all
the force he could, and push Early to the last moment. General Wright
commenced the pursuit on the 13th; on the 18th the enemy was overtaken
at Snicker's Ferry, on the Shenandoah, when a sharp skirmish occurred;
and on the 20th, General Averell encountered and defeated a portion of
the rebel army at Winchester, capturing four pieces of artillery and
several hundred prisoners.

Learning that Early was retreating south towards Lynchburg or Richmond,
I directed that the 6th and 19th corps be got back to the armies
operating against Richmond, so that they might be used in a movement
against Lee before the return of the troops sent by him into the valley;
and that Hunter should remain in the Shenandoah Valley, keeping between
any force of the enemy and Washington, acting on the defensive as much
as possible. I felt that if the enemy had any notion of returning, the
fact would be developed before the 6th and 19th corps