Complex sentence: Lee helped plan the D-Day drops into Normandy, and had trained to jump with his men, but was sent back to the States a few months before the battle due to either a heart attack or a stroke.
Split into simpler sentences: Lee helped plan the D-Day drops into Normandy, and trained to jump with his men. However, he was ordered to the United States several months before the invasion, suffering either a heart attack or a stroke.