Though recruited in Philadelphia, the brigade was made up of four regiments mustered into the Union army as the 1st, 2nd, 3rd, and 5th Californian regiments at the insistence of California senator James A. McDougall. McDougall paid to equip the 1st, rather than have his home state unrepresented in the war; California was too distant to provide troops for a conflict many believed would end quickly. Organized by Edward D. Baker, the other senator from California, the 1st was recruited by Isaac J. Wistar in the spring of 1861, with Baker its colonel and Wistar its lieutenant colonel.

Baker transported the 1st California to New York for its training, and on June 29, the regiment passed through Philadelphia en route to Washington. By October, the 1st had been joined by the 2nd (Fire Zouaves), raised from among the city's fire departments; the 3rd, an almost all Irish regiment; and the 5th, reorganized militia and 90-day men. Baker assumed command of the brigade, leaving Wistar in command of the 1st California.

Edward D. Baker made his reputation for brilliance on the Senate floor, but lost his life because of ineptitude on the battlefield. The British born Whig congressman from Illinois had greatly impressed the young Abraham Lincoln, and on September 21, 1861, through his friendship with Lincoln, he was to command (as a colonel) the 3rd Brigade of Stone's Division in the Army of the Potomac, and on October 3, 1861, he led the brigade into action.

Demonstration

Ordered only to make a "demonstration" along the banks of the Potomac River, Baker saw an opportunity not only for glory, but for a quick boost to his political career. He took his entire brigade across the river to attack a Confederate encampment at Leesburg, Virginia, but foolishly led his men to the top of a hill where they were easy targets for Confederate fire. In the resulting battle of Ball's Bluff on October 21, Baker was killed and his men broke in the confusion as a result of inadequate training and poor leadership; the brigade was driven into the river where many men drowned.

For days, their bodies washed up on the shore of Washington, D.C. (The official inquiry into Ball's Bluff deliberately avoided finding fault with Baker's actions; instead, it made a scapegoat of his divisional commander, General Charles P. Stone. Many believed it was only Baker's friendship with Lincoln that preserved his posthumous reputation.)

With Baker dead, the brigade assumed its identity as the Philadelphia Brigade, with the 1st California being redesignated the 71st Pennsylvania; the 2nd California became the 69th Pennsylvania and the 3rd California became the 106th Pennsylvania.

In March, 1862, Brigadier General William W. Burns, an experienced, competent regular army officer, took over command of the brigade. Designated as the 2nd Brigade/2nd Division/II Corps, it fought on June 29 at Savage Station during the Peninsula Campaign. That September, under Major General Oliver Howard, the brigade formed the third line in Major General John Sedgewick's division during the charge across the cornfield at Antietam. In that disastrous assault, the Philadelphians were mowed down by a raking gunfire from the Confederate ranks - the 71st lost nearly 150 men and the Fire Zouaves counted 220 casualties.

Harper's Ferry

Sent to garrison Harper's Ferry and recuperate, the brigade returned to the line in December and was the first unit to cross the Rappahannock River as Major General Ambrose E. Burnside moved south to check Confederate General Robert E. Lee's invasion of Maryland. Ordered to cover the right flank of the Federal column during the attack on Marye's Heights at Fredericksburg, the brigade became an easy target for Confederate sharpshooters, but was spared from Burnside's devastating attack on Lee's line.

In June, 1863, the brigade marched into Pennsylvania when Lee again invaded the north, and the brigade fought on the Federal right on Cemetery Ridge at Gettysburg. Pitted against Lieutenant-General James Longstreet's troops on July 2, the unit pushed back the Confederate assault, and on July 3, held its line against the furious onslaught of Pickett's Charge. The 69th lost all its field officers and about half of the 258 men engaged; the 72nd counted 189 dead and wounded, and the 71st tallied losses at 25 percent. In two days, the Philadelphia Brigade captured 750 prisoners and three stands of enemy colors.

The fighting at Gettysburg destroyed the brigade as a unit. Brigaded in March 1864 with the 152nd New York, a heavy artillery regiment, the 71st and 72nd were mustered out of service the next month when their enlistments expired. The 106th and 69th merged with other depleted regiments and served until the war ended. In May 1865, they marched in the Grand Review of troops in Washington, D.C. The Philadelphia Brigade was the only Federal unit named for its home city.