Possible Clues:

• 1910s conflict, for short
• 1914-'18 conflict, briefly
• 1910s conflict: Abbr
• What the U.S. joined in Apr. 1917
• It ended on Nov. 11
• The U.S. joined it in 1917
• It ended Nov. 11, 1918
• Gen. Pershing's conflict
• 1914-18 conflict, for short
• It ended on Nov. 11, 1918