Complex sentence: In April 1865, the Confederate regime fled Richmond as U.S. forces approached, as they fled, they set fire to Richmond's public works to prevent them from being used by U.S. forces.
Split into simpler sentences: In April 1865, the Confederate regime fled Richmond as U.S. forces approached the city. As the Confederates fled, they set fire to Richmond's public works to prevent them from being used by U.S. forces.