The Thirteenth Mississippi’s supporting role in the battle of Fort Sanders was preceded by its supporting role in the birth of the Confederate battle flag.
The Seventh Louisiana Regiment led Colonel Jubal Early’s brigade onto the Manassas battlefield on July 21, 1861, with the Thirteenth Mississippi next in the column and the Seventh Virginia bringing up the rear.
And the story goes that Gen. P.G.T. Beauregard mistook the Seventh Louisiana’s Confederate First National flag for the Stars & Stripes of a flanking Union unit and almost ordered a retreat of his forces, which could have swung the battle to the Union.
Instead, Beauregard figured out his confusion in time and the rest is history. Including his decision to create the Saint Andrews Cross battle flag to eliminate future such problems, according to this piece from the Richmond Daily Dispatch of Nov. 27, 1861.
Via Bull Runnings.
As for the battle flag’s continuing modern use as an emblem of valor and/or racism (take your pick), one of the most evenhanded analyses is by historian John M. Coski available here.