Final answer:
The Lusitania was a British passenger liner sunk by a German submarine in 1915. Unrestricted submarine warfare was a strategy used by Germany in World War I that could be problematic.
Step-by-step explanation:
The Lusitania was a British passenger liner that was sunk by a German submarine in May 1915. It was traveling from New York to Great Britain when it was torpedoed off the coast of Ireland. More than 1200 people died in the sinking, including 128 Americans.
Unrestricted submarine warfare was a strategy used by Germany during World War I. It involved the use of submarines to attack and sink enemy ships, including civilian vessels, without warning. This strategy could be problematic because it could lead to the loss of innocent lives and provoke other nations to join the war.