9th Regiment Massachusetts Volunteer Infantry 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 . History . 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 . The Ninth participated in the Battle of Malvern Hill on 1 July 1862 as part of the Peninsular Campaign waged by the Union Army intending to capture the city of Richmond . The Ninth Regiment held the hill and prepared it to withstand a siege by the Confederate Army . Supported by five US Navy gunboats on the nearby James River , and benefiting from favorable geography and extensive defensive preparations , the Union Army withstood repeated charges inflicting heavy casualties on the Southern forces . It was in this battle that Colonel Cass received a mortal wound in his face and mouth that eventually took his life . When Colonel Cass fell in battle , command of the Ninth was assumed by Acting Lieutenant Colonel Hawley . Colonel Hawley was subsequently wounded in the same action , and command fell to Acting Major , Captain O ’ Leary . The unit ’ s casualties were very heavy ; along with losing their two top commanders , roughly half the regiment was put out of action , totaling 166 men . Colonel Cass died in Boston Massachusetts on 12 July 1862 and was buried with full military honors at Mt Auburn cemetery . Today , a statue of the Colonel stands in the Boston Public Garden . Following the death of its commander , Colonel Patrick Robert Guiney took command of the regiment on 4 August 1862 . At this point the Ninth took its place in the newly formed Army of Virginia under the command of General John Pope . The regiment participated in the Second Battle of Bull Run , which resulted in a decisive defeat for the Northern Army . News of the defeat at Bull Run so shook the inhabitants of Washington DC that plans were made to abandon the city before the arrival of the Confederate Army . At his urging , General George Brinton McClellan was given command of the Army of the Potomac and led the army to Antietam Creek near Sharpsburg , Maryland on 17 September 1862 where they successfully engaged General Robert E . Lee ’ s army . The regiment subsequently played supporting roles in the Battle of Fredericksburg and the Battle of Chancellorsville . On 2 – 4 July 1863 the regiment was assigned to hold the strategically important position of Big Round Top during the Battle of Gettysburg . With the help of substantial stone breastworks , the regiment successfully withstood several assaults by the Confederate Army , taking casualties of 15 killed , wounded , or missing . The regiment received orders on 30 April 1864 to meet the enemy under the command of General Ulysses S . Grant . The regiment participated in the Battle of the Wilderness in Virginia which resulted in the wounding of Colonel Guiney in the eye . Command of the regiment fell to Lieutenant Colonel Hanley . Along with Colonel Guiney , the regiment suffered casualties of 138 men . It suffered more severe losses at the Orange Turnpike , losing 78 men killed and wounded . The regiment continued to fight at North Anna River and Bethesda Church near Cold Harbor with light losses . On 10 June 1864 , the Fighting 9th withdrew from the field and was transported to Boston Harbor , arriving on 15 June . The regiment was mustered out of the Army on 24 June 1864 . Following the battle , the Ninth ' s three years of service was completed . Men whose terms of service had not yet expired were transferred to the Thirty - second Massachusetts Veteran Volunteers , and the Ninth returned to Boston on 15 June 1864 . Following a welcoming parade and banquet at Faneuil Hall , the men of the regiment were mustered out in a ceremony on Boston Common on 21 June 1864 and the regiment was disbanded .