SOLUTION: The clue "A Line Meant to Hold" points to the Battle of Ellis Bridge.
This Civil War engagement, fought near West Point, Mississippi, in February 1864, was a significant victory for Confederate forces under General Nathan Bedford Forrest. Forrest's troops successfully defended crossings of the Sakatonchee River (Okolona/West Point Road via Ellis's Bridge being a key point) against a larger Union cavalry column led by General William Sooy Smith. The imagery in the clue of a determined defensive line by a creek ("stacked rails by the creek," "lines held tight") that repelled a strong but ultimately faltering enemy attack ("Their thunder came fast — then faltered... They came to break us — left broken instead") reflects Forrest's successful defense. This victory halted Smith's advance, which was intended to support General Sherman's broader Meridian Campaign.