His appeal was heard by Lieuts. Hazen and Merritt, and he was placed under arrest and kept back to be court-martialed, while the rest of his class, (excepting such as had already resigned to join the Southern army) departed for active service. The court-martial was however cut short, through the exertions of his fellow cadets at Washington, by a telegraphic order summoning him there.
Custer reported to the Adjutant-General of the Army at Washington, July 20th, and was by him introduced to Gen. Scott. The company (G, 2nd Cavalry) to which he had been assigned, with the rank of 2nd lieutenant, was at this time near Centerville, and as he was to join it, Gen. Scott entrusted to him some dispatches for Gen. McDowell who commanded the troops in the field. A night's ride on horseback took him to the army, the dispatches were delivered, and then he joined his company before daybreak just as they were preparing to participate in the battle of Bull Run. In this battle, however, the cavalry took but little part; in the frantic retreat that followed, Custer's company was among the last to retire, and did so in good order, taking with them Gen. Heintzelman who was wounded.
After Gen. McClellan took command of the army, Custer's company was attached to Gen. Phil Kearny's brigade, and that general detailed Custer as his aid-de-camp, and afterwards as assistant adjutant-general, which position he held till deprived of it by a general order prohibiting officers of the regular army from serving on the staffs of volunteer officers.
About this time he obtained leave of absence on account of ill health, and visited his sister, Mrs. Reed, at her home in Monroe, Michigan; and it is said that through her entreaties and influence he then gave up the habit of using strong drinks, which, in common with many of his fellow officers, he had acquired during his brief army life near Washington. Thenceforth, through the remainder of his life, he drank no intoxicating liquor.
Returning to the army in Feb. 1862, he was assigned to the 5th Cavalry, and when the enemy evacuated Manassas he participated in the advance on that place, and led the company which drove the hostile pickets across Cedar Run.
When the Army of the Potomac was transferred to the Peninsula, Custer's company was among the first to reach Fortress Monroe, and it then marched to Warwick. Here he was detailed as assistant to the chief engineer, on Gen. W.F. Smith's staff; he served in that capacity during the siege of Yorktown, and planned the earthwork nearest the enemy's lines. At the battle of Williamsburg, where he acted as aid-de-camp to Gen. Hancock, he effected the capture of a battle-flag—the first taken by the Army of the Potomac.
When the army was encamped near the Chickahominy River, late in May, Custer accompanied Gen. Barnard, the chief engineer of the army, on a reconnoisance outside the picket line to the bank of the river; and at the request of his superior, he dismounted, jumped into the river, and waded across the stream—the object being to ascertain the depth of the water, which in some places came nearly up to his shoulders. On reaching the opposite bank he examined the ground for some distance, and discovered, unseen by them, the position of the enemy's pickets. Barnard reported to McClellan that the river was fordable, and how he had ascertained that it was so. McClellan sent for Custer, and was so pleased with his appearance and courageous act that he transferred him to his own staff; and in June, Custer received from the Secretary of War his appointment as additional aid-de-camp, with the rank of captain during the pleasure of the President. Previously to this he had crossed the Chickahominy at daybreak with a company of infantry, attacked the enemy's picket post, and captured prisoners and arms.
Custer served on McClellan's staff through all of the Peninsular campaign; and after the battles of Gaines' Mills, Fair Oaks, Malvern Hill, etc., retreated with him to the protection of the gunboats at Harrison's Landing on the James River. Subsequently, after the withdrawal of the army from the Peninsula and the defeat of Banks and Pope in Virginia, he was McClellan's aid-de-camp in the Maryland campaign which closed with the battle of Antietam. When McClellan was superseded by Burnside, Nov. 10th, 1862, Custer accompanied his chief to Washington, and subsequently visited his friends in Ohio and Michigan. His staff position as captain ceased with the retirement of McClellan, and he was now a first lieutenant, commissioned July 17th, 1862.
In April, 1863, Custer rejoined his company which was with Gen. Hooker's army near Fredericksburg, and took part in the battle of Chancellorsville. In June he was on the staff of Gen. Pleasonton, then chief of the cavalry corps, and was conspicuous at Beverly Ford and other places across the Rappahannock where Stuart's cavalry were met and roughly handled.
At the battle of Aldie, Virginia, Custer distinguished himself in the charge made by Kilpatrick's cavalry. The onset was irresistible; the Confederate forces were driven back in confusion, and Custer's impetuosity carried him far within their lines, from which he was allowed to escape in consequence, he believed, of the similarity of his hat to those worn by the Confederates. For his gallantry in this action, Custer was promoted at one bound from a first lieutenant to a brigadier-general.