The man had a talent for bringing disaster on himself on everyone around him. Due to his failure to follow orders and to place his troops in a secure position on the field, his II Corps suffered a terrible pummeling at Gettysburg. As the America's Civil War article pointed out, the II Corps had to be disbanded in 1864.
The man was also a menace to himself. He only murdered a man before the war. It took the Johnny Cochran of the era, Edwin Stanton, to get him off. He later marched up to the thick of the fighting at Gettysburg, over the advice of his staff. He lost his leg as a result.
The only good thing Dan Sickles ever did in his life was to preserve Gettysburg as federal land. He was little more than a egocentric, grandstanding politician.
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