the battle of south mountain was fought on september 14, 1862 in maryland’s south mountain between confederate and union forces. the battle of south mountain was a very important battle. it was the first major battle fought north of the potomac river. it was the first invasion of the northern united states by a confederate army. it was also here and not at antietam where the confederates under general robert e. lee were defeated and turned back. the day-long pitched battle was fought for control of the three passes through the mountain: crampton's, turner's, and fox's gaps.