In September he became aide-de-camp to General John Sedgwick and served with him in the Peninsula Campaign of early 1862 until falling ill with typhoid fever.
After recuperating, he returned to duty in August 1862 and again worked as an aide-de-camp, this time to General-in-Chief of the Union Army, Henry Wager Halleck.
He served with Sedgwick for just over a year and took part in a series of battles, including Gettysburg, Rappahannock Station, Raccoon Ford, Mine Run, and the Wilderness.
On that day, at the Harpeth River in Tennessee, he took command of the 4th Cavalry Regiment and led a successful mission to capture a Confederate artillery battery.
During the Red River War, he fought with Colonel Ranald S. Mackenzie in the Battle of Palo Duro Canyon, where he led the 4th Cavalry's leading battalion.
[2] His oldest daughter from his first marriage, Natalie Sedgewick Beaumont, married George Alexander Forsyth, a fellow 4th Cavalry officer who was only three or four months his junior.