Leaving the State, August 2, 1861, it proceeded to Cockeysville, Md., where it guarded the railroad for several weeks. It sailed for Hatteras Inlet, N. C., September 24th, and thence in November to Fort Monroe, where it passed the winter. While encamped at Newport News, it participated in the fight between the Merrimac and the Congress; the regiment having been deployed on the beach under the fire of the Confederate vessels, prevented the enemy from taking possession of the Congress. It went to Norfolk in May, and in the following month joined McClellan's Army — then on the Chickahominy, where it was placed in Robinson's (1st) Brigade, Kearny's (3d) Division, Third Corps. In the affair of June 25, 1 862, at Oak' Grove, or "The Orchards," the Twentieth received the principal attack and sustained the heaviest loss, its casualties amounting to 11 killed, 82 wounded, and 32 missing. At Manassas, the losses were 4 killed, 35 wounded and 6 missing, Colonel Brown being among the killed. After this battle the Corps, having become greatly reduced by its losses, was withdrawn from active service to enable it to rest, and the wounded or missing to return; hence it was not present at Antietam. At Gettysburg — Ward's Brigade, Birney's Division— its losses amounted to 32 killed, including Colonel Wheeler, 114 wounded, and 10 missing. In 1864, the division was transferred to the Second Corps, the Twentieth fighting under Hancock from that time on. Lieutenant-Colonel Meikel was killed at Petersburg.