9th Regiment Massachusetts Volunteer Infantry

9th Regiment Massachusetts Volunteer Infantry
Statue of Colonel Thomas Cass standing in the Boston Public Garden
Active	11 June 1861–21 June 1864
Country	United States of America
Allegiance	Union
Branch	United States Army
Type	Infantry
Part of	In 1863: Sweitzer's Brigade, Barnes's Division, V Corps, Army of the Potomac
Motto(s)	"The Fighting Ninth"
Commanders
Notable commanders	Col. Thomas Cass Col. Patrick Robert Guiney
Insignia
V Corps badge

The 9th Regiment Massachusetts Volunteer Infantry was a military unit from Boston, Massachusetts, USA, part of the Army of the Potomac during the American Civil War. It is also known as "The Fighting Ninth". It existed from 1861 to 1864 and participated in several key battles during the war. The unit is an Irish heritage unit, with many volunteers having been born in Ireland.

The Ninth Regiment was created on 11 June 1861 under the command of Colonel Thomas Cass in Boston recruiting primarily Irish-Americans. Initial funding for the regiment came from Patrick Donahoe, publisher of The Boston Pilot. Initially sheltered at Boston's Faneuil Hall, they later camped at Long Island in Boston Harbor.

On 30 June 1861, the unit arrived in the Washington, D.C. vicinity and was welcomed by President Abraham Lincoln. They remained in the vicinity of Arlington Virginia performing picket duty and built a fort on the Potomac River called Fort Cass after their commanding officer. As a unit of the Army of the Potomac under the command of Major General George B. McClellan, the regiment moved south with the rest of the Army in pursuit of the confederate forces.

On 26 June 1862 the regiment participated in the Battle of Mechanicsville, about six miles from Richmond Virginia which resulted in a Union Army victory and more than 2,000 Confederate casualties. The next day the regiment was assigned to hold the bridge over the mill creek in the action that later became known as the Battle of Gaines Mill. The regiment was told to hold the bridge pending the arrival of two supporting regiments. The support troops were delayed in their arrival new and the Ninth Regiment successfully repelled several Confederate assaults on the bridge. The supporting troops, including a regiment of New York Irishmen, the 63rd New York eventually arrived, but by this point the opposing forces had built their strength and the Ninth was unable to hold the bridge. Eventually the Confederate forces led by General James Longstreet and General Ambrose Powell Hill, broke the Union line forcing the Union Army into retreat. The Ninth Regiment brought up the rear of the retreat fighting a rearguard action against the opposing forces. The Union Army lost 6,000 killed or wounded in this battle and the Ninth’s casualties totaled 252 men.