Complex sentence: In early 1643, Parliament placed Newark under siege, and the garrison at Ashby was sent to relieve it; forming part of a larger force under Prince Rupert, they forced the surrender of the Parliamentary force.
Split into simpler sentences: In early 1643, Parliament placed Newark under siege, and the garrison at Ashby was sent to relieve it. As part of a larger force under Prince Rupert, they forced the surrender of the Parliamentary force.