Fighting began on April 12, 1861, when local militia attacked United States troops at Fort Sumter in South Carolina.

[2] In response to Lincoln's call for volunteer troops, Companies A through K (there was no J) were mustered into the 18th Pennsylvania Cavalry Regiment from August through November.

[Note 2] Captain James E. Gowen of Company E was promoted to lieutenant colonel on November 25, 1862, and was the regiment's highest-ranking officer over the next few months.

[6] In early December 1862 the regiment moved by rail to the Bladensburg, Maryland area, close to Washington, D.C.[6] They received training at this location, and were armed with a saber and Merrill carbine.

A West Point graduate, he had been an officer in the regular U.S. Army, and had previously been a lieutenant colonel with the 12th Massachusetts Infantry Regiment.

[9] Camp for the 18th Pennsylvania Cavalry was moved to Virginia on January 1, 1863, and settled a week later on the Little River Turnpike about 2 miles (3.2 km) from Fairfax Courthouse.

[5][Note 3] Historian Robert W. Black observes "the 18th Pennsylvania would become a fine regiment", but "in January 1863, it was a collection of civilians in uniform, poorly equipped and armed.

[16] When the two companies were added, the regiment became part of Colonel Richard Butler Price's Independent Cavalry Brigade, XXII Army Corps, Department of Washington.

[18] The regiment continued to show its inexperience on March 1 when Major Joseph Gilmore led 200 men on a westward reconnaissance toward Aldie, Virginia.

[21][Note 6] In April, the regiment became part of the Third Brigade, Brigadier General Julius Stahel's Cavalry Division, XXII Army Corps.

[30] Its mission was to prevent Confederate cavalry under the command of Major General James Ewell Brown "Jeb" Stuart from joining the rest of Lee's army.

[33][Note 7] A small group of "less than a dozen" men, led by Captain Thadeus Freeland of Company E, protected the right flank as it moved a few miles east of the road to Hanover.

[35] Not far from Gitt's Mill, Freeland's men and an enemy scouting party from the 13th Virginia Cavalry Regiment spotted each other, and a long-range shot killed one of the Confederates—the first casualty of the Battle of Hanover.

[37] When Potter and his men were about 1 mile (1.6 km) from Hanover, they found their path to town blocked by the same group of about 60 Confederates who demanded their surrender.

[44] After the Battle of Gettysburg, Lee's army retreated toward Williamsport, Maryland, where it planned to cross the Potomac River to the relative safety of Virginia.

As Meade's army pursued Lee, several battles and skirmishes occurred—including a fight by Kilpatrick's division in the mountains at Monterey Pass where a Confederate wagon train was captured.

[57] Both portions of Lee's army, the wagon train of wounded men and the healthy soldiers, needed to pass through Hagerstown, Maryland, on their separate routes to Williamsport.

[58] Most of the 18th Pennsylvania Cavalry's fighting took place in Hagerstown, which is located on the National Road six miles (9.7 km) from the Potomac River.

[89] He discovered a wagon track north of a road called Roberson's Run that led east and began a retreat with Chapman leading and Bryan in the rear.

[90] The 18th Pennsylvania Cavalry, commanded by Brinton, again protected the rear and was ordered to hold for one half hour before attempting to rejoin Wilson.

Over the next few days, Sheridan moved by White Oak Swamp to Malvern Hills where they were accidentally shelled for a brief time by Union gunboats on the James River.

[105] Sheridan reported on May 20 that he found "little subsistence and forage" at White House landing (on the river) and wanted "ammunition first and supplies of all kinds".

That portion of the regiment retreated in confusion, but entrenched United States Colored Troops who had been sent to support McIntosh stopped the enemy.

[114] The regiment, under Brinton's command, advanced in a move toward White Oak Swamp on June 15, and had had to fight infantry in a wooded area while unassisted.

Lieutenant Samuel McCormick was killed and now-Captain Tresonthick was mortally wounded on that day at St. Mary's Church in an engagement that lasted nearly five hours.

[124] On July 31, 1864, Grant decided to have Sheridan be his field commander to fight Confederate Lieutenant General Jubal Early in the Shenandoah Valley.

[140] The division continued destroying crops, barns, and mills in the valley as it moved north, which depleted the food and infrastructure used to supply the Confederate Army.

"[143] Custer reported "Never since the opening of this war had there been witnessed such a complete and decisive overthrow of the enemy's cavalry" and that a pursuit was made "vigorously for nearly twenty miles.

[145] The 18th Pennsylvania Cavalry was assigned the task of supporting a battery that was exchanging fire with enemy artillery, and suffered some casualties when a shell burst directly over the regiment.

[161] On March 7 during the trip north, Rosser's cavalry attacked the Union force at Rude's Hill near Mount Jackson.