Complex sentence: Union forces had insufficient time to clear the area known as the Wilderness, a tangle of scrub brush and undergrowth in which part of the Battle of Chancellorsville had been fought the previous year.
Split into simpler sentences: Union forces had insufficient time to clear the area known as the Wilderness. The Wilderness was a tangle of scrub brush and undergrowth in which part of the Battle of Chancellorsville had been fought the previous year.