They got to know with the photographs from the U - 2 spy plane.
<em>Hope this helped! :)</em>
it was used in the balkans war.
Answer:
April 6,1917
Explanation:
the U.S. joined its allies--Britain, France, and Russia--to fight in World War I. Under the command of Major General John J. Pershing, more than 2 million U.S. soldiers fought on battlefields in France. Many Americans were not in favor of the U.S. entering the war and wanted to remain neutral
