The United States entered World War I after the RMS Lusitania was attacked by several of these vehicles during a time when their use during warfare was "unrestricted."
Some British examples of these objects were designated "K-class," and had a tendency to fail or otherwise get destroyed. One of these objects, which was supervised by Hyman Rickover, was used to carry out Operation Sunshine. During World War II, some of these vehicles traveled together in "wolf packs" and were part of the Kriegsmarine. Early American examples of them included the Turtle and the Hunley, and the Nautilus was the first nuclear-powered one. The United States entered World War I after the RMS Lusitania was attacked by several of these vehicles during a time when their use during warfare was "unrestricted." For 10 points, name these underwater vehicles exemplified by the German U-boat.
Some British examples of these objects were designated "K-class," and had a tendency to fail or otherwise get destroyed.
submarine [or submersible; or U-boat or Unterseeboot before they are read; prompt on "sub"]