Civil War Union Army Officer. He began his Union Army service when he enrolled as a Private in Company C, 2nd New Jersey Militia on April 26, 1861. He served in the defenses of Washington, DC, and in the July 1861 First Bull Run Campaign (where his regiment was held in reserve), and was honorably mustered out on July 31, 1861. He then enlisted in the newly raised three-year enlistment 7th New Jersey Volunteer Infantry, being mustered in as a Sergeant in Company F on September 2, 1861. He served in this role through the Spring 1862 Peninsular Campaign, participating in the Battles of Williamsburg, Fair Oaks, The Seven Days, Second Bull Run and the Antietam Campaign. On October 2, 1862 he was promoted to Sergeant Major of the regiment, filling the vacancy when Charles R. Dougherty was advanced to 2nd Lieutenant of Company A. He served as the 7th New Jersey's top enlistment man through the December 1862 Fredericksburg Campaign, and the January 1863 "Mud March". On April 13, 1863 Company C's 2nd Lieutenant, Henry Crane, was promoted to 1st Lieutenant, and Sergeant Major Fosselman was commissioned as a 2nd Lieutenant to fill the vacancy. He served through the May 1863 Battle of Chancellorsville, the July 1863 Battle of Gettysburg, and the Fall 1863 Mine Run Campaign. then again followed the advancement of Henry Crane. When that officer was promoted to Captain and commander of Company C, Charles Fosselman was promoted to 1st Lieutenant (January 1864). He led his men in the terrible combat that marked General Ulysses S. Grant's May 1864 Overland Campaign, fighting in the Battles of the Wilderness, Spotsylvania and the siege of Petersburg. When Charles R. Dougherty, who by this time had become the regimental Adjutant, was severely wounded in action and mustered out due to those wounds in September 1864, Lieutenant Fosselman was appointed as Adjutant to take his place. He performed this duty in the continuing operations aimed at capturing Petersburg, Virginia, and was responsible for mustering in the re-enlisting veterans of the 5th New Jersey Infantry, who were transferred to the 7th New Jersey in the fall of 1864 when that regiment's enlistment expired. Finally, on the eve of the final assaults on Petersburg, he was promoted to Major of the regiment when the previous Major, Daniel Hart, became the 7th New Jersey's fourth Lieutenant Colonel (March 20, 1865). He served through the conclusion of the conflict, having been promoted five times. He was honorably mustered out of Federal service on July 17, 1865.