Complex sentence: Diem threw himself into preparations for the 1916 games; his principal partner in this and most of his Olympic endeavors was Theodor Lewald, who would for many years be chairman of the German Olympic Committee.
Split into simpler sentences: Diem threw himself into preparations for the 1916 games. His principal partner in this and most of his Olympic endeavors was Theodor Lewald, who was for many years chairman of the German Olympic Committee.