Initially composed of the 2nd, 6th, and 7th Wisconsin and the 19th Indiana regiments, the Iron Brigade was organized at Washington, D. C. in the fall of 1861. Brig. Gen. Rufus King, former editor of Milwaukee's Sentinel and Gazette served as its first commander until Brig. Gen. John Gibbon assumed command in May 1862. A regular Army officer, Gibbon was keen on training and discipline and did much to improve the brigade's efficiency. To bolster morale and foster a sense of esprit de corps, Gibbon outfitted them with the black felt Army hat and canvas gaiters. The hats in particular became a source of pride and ultimately provided the distinctive sobriquet of the "Black Hat Brigade."
The Brigade saw its first serious action at August 28, 1862, at Groveton, VA where, along with Battery B, 4th US Artillery, it destroyed a third of the attacking Stonewall Brigade. Groveton was costly for the Iron Brigade as well resulting in 751 casualties for the Mid-westerners. A few weeks later, at South Mountain, the brigade lost another 318 men. It was there that Maj. Gen. Joseph Hooker referred to the hard-fighting Westerners as his "iron brigade."
A few days later at Antietam, as a part of Hooker's I Corps, the Iron Brigade saw fierce action in the west woods and in Miller's cornfield. A determined charge momentarily displaced Jackson's surprised Confederates, but a blistering counterattack by Gen. Hood's division forced the Iron Brigade back through the cornfield and into the west woods. Resultant losses for the Iron Brigade amounted to 348 men.
After Lee's failed Maryland Campaign, the Iron Brigade was strengthened with the addition of the 24th Michigan. It remained a Western brigade amongst a sea of Eastern units, and no doubt served as another source of pride for its battle hardened veterans. The brigade saw limited action at Fredericksburg and Chancellorsville but it bore a heavy burden at Gettysburg.
On the first day's fighting, the brigade was deployed on McPherson's and Seminary Ridge. In their dogged defense of that ground, the Iron Brigade suffered enormous casualties but managed to slow the Confederate advance thus enabling the timely deployment of fresh federal troops. Casualties for the brigade numbered approximately 1,212 out of 1,883 effectives. The 24th Michigan lost 80% of its number, while the 2nd Wisconsin suffered 77% casualties. The battle at Gettysburg essentially destroyed the original Iron Brigade. Although the name continued in use, the brigade lost some of its original character. The 2nd Wisconsin was disbanded in July 1864, and Eastern regiments were added to the brigade, altering its formerly all-Western composition. However the Sixth and Seventh Wisconsin Infantry units served until the end of the war earning the honored "Veteran Infantry" designation.