100th Pennsylvania Infantry in the American Civil War

Overview

The 100th Pennsylvania Infantry (Three Years’ Service), also known as the “Round Head Regiment,” served as an infantry unit for the Union Army during the American Civil War. Raised primarily in the southwestern counties of Pennsylvania, the regiment was organized in August 1861 and mustered into service at Washington, D.C., for a three-year term. The 100th Pennsylvania Infantry participated in major campaigns across the Eastern and Western Theaters, earning distinction for its service and heavy combat losses.

This regiment is distinct from any other Pennsylvania infantry units with similar designations or service terms.

Organization & Service

The 100th Pennsylvania Infantry (Three Years’ Service) began organizing in August 1861, with twelve companies recruited, though one company was later transferred to the 105th Pennsylvania Infantry. The regiment was ordered to Washington, D.C., on September 2, 1861, where it completed its organization and mustered into U.S. service. Shortly after, it was sent to Fortress Monroe and embarked on the Port Royal Expedition, arriving with the fleet on November 7, 1861, and participating in the capture of Forts Walker and Beauregard. The regiment then occupied Beaufort, South Carolina, remaining in the area for several months, during which it suffered significant losses from sickness.

In June 1862, the 100th took part in operations against Charleston, notably suffering heavy casualties at the engagement near Secessionville on June 16. After returning to Hilton Head and Beaufort in July, the regiment was ordered to Virginia, joining the 9th Corps of the Army of the Potomac. It participated in the Second Battle of Bull Run in August 1862, where it endured severe losses, and was subsequently engaged at Chantilly and South Mountain. Due to its depleted numbers, the regiment was held in reserve at Antietam. It fought at Fredericksburg in December 1862 and was selected to cover the withdrawal of Union forces, recognized as the most reliable skirmish regiment in its brigade.

In March 1863, the 100th Pennsylvania was transferred to the Department of the Ohio and sent to Lexington, Kentucky. In June, it moved to support General Grant at Vicksburg, Mississippi, and later participated in the fighting at Jackson, Mississippi, suffering additional losses from both combat and disease. The regiment was then ordered to East Tennessee, where it engaged at Blue Springs and endured the hardships of the Knoxville siege. Despite these challenges, nearly the entire regiment reenlisted on January 1, 1864.

After reporting to Annapolis, Maryland, in March 1864, the 100th became part of the 2nd Brigade, 1st Division, 9th Army Corps, Army of the Potomac. The regiment was heavily engaged in the Overland Campaign, fighting at the Wilderness, Spotsylvania, North Anna River, and Cold Harbor. It then participated in the Petersburg Campaign, including the Battle of the Crater, the Weldon Railroad raid, Poplar Spring Church, Hatcher’s Run, and the final assault on Petersburg on April 2, 1865. The 100th Pennsylvania Infantry was mustered out of service at Washington, D.C., on July 24, 1865.

Engagements & Campaigns

• Port Royal Expedition (November 1861)
• Operations against Charleston, including Secessionville (June 1862)
• Second Battle of Bull Run (August 1862)
• Chantilly (September 1862)
• South Mountain (September 1862)
• Antietam (held in reserve, September 1862)
• Fredericksburg (December 1862)
• Vicksburg Campaign (June–July 1863)
• Jackson, Mississippi (July 1863)
• Blue Springs (October 1863)
• Siege of Knoxville (November–December 1863)
• Wilderness (May 1864)
• Spotsylvania (May 1864)
• North Anna River (May 1864)
• Cold Harbor (June 1864)
• Petersburg Campaign, including the Battle of the Crater, Weldon Railroad, Poplar Spring Church, Hatcher’s Run, and final assault (June 1864–April 1865)

Casualties

According to Fox’s Regimental Losses and state records, the 100th Pennsylvania Infantry suffered significant casualties:

• Killed or mortally wounded: 11 officers, 205 enlisted men
• Died of disease: 2 officers, 142 enlisted men
• Total deaths: 360

Figures are based on official reports; some variations may exist in different sources.

Field Officers & Commanders

• Colonel Daniel Leasure
• Colonel N. J. Maxwell
• Lieutenant Colonel James Armstrong
• Lieutenant Colonel David A. Leckey
• Lieutenant Colonel Matthew M. Dawson
• Lieutenant Colonel Joseph H. Pentecost
• Lieutenant Colonel Charles Wilson
• Major David A. Leckey
• Major Matthew M. Dawson
• Major James Armstrong
• Major James H. Cline
• Major T. J. Hamilton
• Major David Critchlow
• Major N. J. Maxwell
• Major James W. Bard

Regimental Roster

The full roster of the 100th Pennsylvania Infantry (Three Years’ Service), including officers and enlisted men, is available for genealogical and historical research. The roster provides names, ranks, and service details as recorded in official state and federal records.

To view the complete roster, visit the 100th Pennsylvania Infantry (Three Years’ Service) Roster page.

Sources & References

• Official Records of the Union and Confederate Armies, Series I, Volumes VI, IX, XII, XXI, XXXVI, XL, XLVI
• Pennsylvania Adjutant General’s Report, 1861–1865
• Dyer, Frederick H., A Compendium of the War of the Rebellion
• Fox, William F., Regimental Losses in the American Civil War
• The Union Army, Federal Publishing Company, 1908, Volume 1
• Contemporary wartime reports and regimental documents