Complex sentence: The D-Day Normandy landings on June 6,1944, were costly but successful; a month later the invasion of Southern France took place, and control of the forces which took part in the southern invasion passed from the AFHQ to the SHAEF.
Split into simpler sentences: The D-Day Normandy landings on June 6,1944, were costly but successful. A month later, the invasion of Southern France took place, and control of forces in the southern invasion passed from the AFHQ to the SHAEF.