Complex sentence: They arrived and began their attack on March 2nd, the time of their attack was fortunate because they arrived just when the wizard Saruman was emptying Isengard for a final assault on Rohan.
Split into simpler sentences: They arrived and began their attack on March 2nd. The time of their attack was fortunate, because they arrived just when the wizard Saruman was emptying Isengard for a final assault on Rohan.