Complex sentence: Walker enlisted in the United States Army in Denver, on 15 December 1917 and undertook flying training at the University of California's School of Military Aeronautics and at the pilot training base at Mather Field, near Sacramento, California.
Split into simpler sentences: Walker enlisted in the United States Army in Denver, on 15 December 1917. He received flight training at the University of California's School of Military Aeronautics and at the pilot training base at Mather Field, near Sacramento, California.