Complex sentence: In April 1917 the United States entered World War I. Shortly afterwards, Scott, then 19 years old, joined the Army and served in France as an artillery observer with the 2nd Trench Mortar Battalion, 19th Field Artillery.
Split into simpler sentences: In April 1917, the United States entered World War I and shortly afterwards, Scott, then 19 years old, joined the Army. He served in France as an artillery observer with the 2nd Trench Mortar Battalion, 19th Field Artillery.