Complex sentence: The 1st Marine Division, then under the command of Major General James Mattis, was one of the two major U.S. land forces that participated in the 2003 invasion of Iraq as the land component of the 1st Marine Expeditionary Force.
Split into simpler sentences: The 1st Marine Division, then under the command of Major General James Mattis, was one of the two major U.S. land forces that participated in the 2003 invasion of Iraq. The division was the land component of the 1st Marine Expeditionary Force.