The use of submarines during world war i resulted in

the sinking of passenger liners.
a quick end to the war.
the poisoning of water supplies.
greater support for air attacks.

I believe the answer is: the sinking of passenger liners.

The most notable one was The sinking of the Cunard ocean liner RMS Lusitania which happened on Friday, 7 May 1915 .

The ship belonged to united kingdom. Since they are united states' ally , this event force United States to declared war to Germany and join the world war I.

answer; the federal reserve system;