As Gen. William Tecumseh Sherman swept through Georgia in 1864, he fought several small battles against an ever-retreating Gen. Joseph E. Johnston who had replaced the beleaguered Gen. Braxton Bragg as leader of the Confederate Army of Tennessee. After heavy rains slowed Sherman’s advance, Johnston’s army entrenched along the Brushy Mountain line. Hemmed in by the mountains and impassable roads, Sherman noted in his reports to Washington, “Kennesaw is the key to the whole country.” Ultimately, Sherman would outflank Johnston and grind down his army’s defenses with a brazen frontal...