The Iron Brigade was part of the Union Army of the Potomac made up from men from three Western states, now part of the Midwest, that fought entirely in the Eastern Theater. The Brigade initially consisted of the 2nd, 6th, and 7th Wisconsin Volunteer Infantry Regiments, the 19th Indiana, and Battery B of the 4th U.S. Light Artillery. Later they were joined by the 24th Michigan Infantry. Noted for its strong discipline, its unique uniform appearance, and its tenacious fighting ability, the Iron Brigade suffered the highest percentage of casualties of any brigade in the war. They became known as The Black Hat Brigade because they universally wore the Model 1858 Hardee hat rather that the blue forage caps worn by most of the other Union Army units in the Army of the Potomac. Because of their performance at The battle of South Mountain in September of 1862 they earned the nickname The Iron Brigade of the West, or more commonly, The Iron Brigade.