Complex sentence: The Union line, initially bolstered by the infantry, briefly seized the momentum and attacked the Confederate left flank, but Forrest launched an attack from his extreme right and left wings, before the rest of the federal infantry could take to the field.
Split into simpler sentences: The Union line, initially bolstered by the infantry, briefly seized the momentum and attacked the Confederate left flank, but Forrest had other plans. He launched an attack from his extreme right and left wings before the rest of the federal infantry could take to the field.