Complex sentence: An alternative might have been to run the batteries, probably at night as would be done successfully in the 1863 Vicksburg Campaign; once past the fixed river batteries, Fort Donelson would have been defenseless.
Split into simpler sentences: An alternative might have been to overrun the batteries, probably at night as would be done successfully in the 1863 Vicksburg Campaign. Once past the fixed river batteries, Fort Donelson would have been defenseless.