Complex sentence: At the outset of the U.S. entry into World War I, then-Major General Pershing promoted Patton to the rank of captain and later assigned him to the newly formed United States Tank Corps.
Split into simpler sentences: At the outset of the U.S. entry into World War I, then-Major General Pershing promoted Patton to the rank of captain. Pershing assigned him to the newly formed United States Tank Corps.