What did President Wilson threaten if the Unrestricted Submarine Warfare continued?

Answer 1

Wilson sent 3 notes to the German leadership, which at the time was considered a pretty weak response considering that 139 Americans were aboard the RMS Lusitania when it was sunk by the German U-Boat U-20 on May 7, 1915. The Lusitania was sunk during Germany's initial policy of Unrestricted Submarine Warfare (attacking any vessel considered hostile, regardless of being a warship or not, without provocation or warning).

Though Britain fully expected America to declare war on Germany, Wilson's response was far less than that. In Philadelphia on May 10, 1915, his initial response was:

"There is such a thing as a man being too proud to fight. There is such a thing as a nation being so right that it does not need to convince others by force that it is right"

In 3 separate communications to Germany, he made clear his position on the matter of URSW:

1. Affirming the right of Americans to travel as passengers on merchant ships and called for the Germans to abandon submarine warfare against commercial vessels, whatever flag they sailed under.

2. Rejecting German arguments that the British blockade was illegal, and was a cruel and deadly attack on innocent civilians, and their charge that the Lusitania had been carrying munitions.

3. An ultimatum, to the effect that the US would regard any subsequent sinkings as "deliberately unfriendly".

The German leadership changed their policy to one of Restricted Submarine Warfare, a decision that hampered their efforts to defeat the British Royal Navy's blockade efforts. When Germany reversed its position in 1917, it was the catalyst that brought the U.S. into WWI.