The sinking of the RMS Lusitania by a German U-Boat in 1915 was at the time a very controversial event. It, along with the Zimmermann Telegram, paved the way for the entry of the United States into World War I. Even today, there is debate over the event, specifically about what exactly the ship was carrying (besides some 128 Americans), possible foreknowledge, and the alleged destruction of the wreck (it also depends on what you mean by "paved the way" for the entry of the United States into the First World War: that country did not join the allied side until 1917, two years after the Lusitania was sunk, and Germany's decision to resort to unrestricted submarine warfare is a more likely candidate for the set of events that qualify as "triggers" for US involvement in that conflict).
The Lusitania was built and launched as an ocean liner in 1907 and at the time of her sinking, the North Sea and the area around the British Isles was declared a war zone. Ships in that area were subject to being torpedoed
if the U-boat crews were bored if they were determined to be non-neutral vessels, and the German Embassy in the U.S. published notices in major American newspapers warning potential passengers of this fact. As the ship was supposed to head to Liverpool, the crew was given instructions on how to avoid being sunk by German subs. When the Lusitania left New York with her passengers, she was in fact, also carrying ammunition.. This, as the Germans insisted, made the ship a legitimate target under the laws of war.
On 7th May, 1915, the Lusitania, by coincidence  was spotted by a German U-boat off the coast of Ireland. The captain of the U-boat decided to give the order to fire a torpedo at the Lusitania, which struck the hull and caused a second explosion, sinking the ship and killing 1,195 people.
Controversy and conspiracy
The sinking of the Lusitania is viewed by conspiracy theorists similarly to the Pearl Harbor attack. It is their belief that the ship was meant to be sunk in order to bring the United States into the war. One of the main pieces of evidence cited is found in a letter to the British Board of trade from Winston Churchill reading,
“”"It is most important to attract neutral shipping to our shores in the hope especially of embroiling the United States with Germany . . . . For our part we want the traffic — the more the better; and if some of it gets into trouble, better still.
This might sound like a smoking gun, but the Lusitania was hardly "neutral shipping". Another trope of this brand of conspiracy is to point to the lack of protection the vessel had during this voyage, as the Lusitania was heavily escorted into Liverpool bay.  The first thing to consider is that governments cock up sometimes, and Britain during World War I was definitely no exception. The government would have had to have known the positions and bearings of the U-boats in the area almost constantly to ensure such an event would happen, which, considering the range of the torpedo and the accuracy of British intelligence at the time, would be impossible. The U-boat crew would have to be relied on to make the decision to fire on the Lusitania in the first place and not miss Exacerbating these practical difficulties, the Royal Navy was extremely slow to implement a convoy system for most commercial shipping, thinking (erroneously, as it turned out) that lone merchant ships would more easily avoid the U-boats and that this was a better means of safety than a convoy escorted by warships. Hence there is no evidence to suggest that there was a government conspiracy; it would have required elaborate planning and foresight and the Lusitania wasn't treated any differently from any other merchant ship at the time, not to mention that her aforementioned speed would probably have made her a low priority for protection in a convoy, even had such a system been in place at the time (in both World Wars, the Royal Navy had problems getting enough escort vessels to protect the convoys, leaving many merchant vessels under- or unprotected).
Destruction of the wreck
Divers examining the wreck have reported that it appears to have been depth charged and mined, with some unexploded ordnance also found. Conspiracy theorists say this is evidence of a cover-up by the British Government. This is ridiculous. The wreckage was likely bombarded to reduce its signature as a reference point for German subs in World War II, as well as to destroy a potential hiding place for them.
- New York Times, 1915. http://query.nytimes.com/mem/archive-free/pdf?res=9E0CE4DE1F3EE733A0575AC0A9639C946496D6CF
- OR WAS IT??????
- Thomas, Andy. Conspiracies: The Facts, the Theories, the Evidence. London: Watkins, 2013. Print. Conspiracies: The Facts. The Theories. The Evidence
- Layton, J. Kent (2010). Lusitania: an illustrated biography. Amberley Books. ISBN 978-1-84868-834-6.
- The Lusitania was faster than the U20 by a good 15 knots. see here and here for specifications
- As seen on this show: http://tv.yahoo.com/shows/treasure-quest/