As a slight note, the Lusitania was not a ship of the Royal Navy; this means it doesn't have the prefix HMS. Instead, it was a Royal Mail Steamer, or RMS. She was an entirely civilian-manned ship, and carrying a large amount of civilian passengers. However, she was also carrying military cargoes, primarily ammunition. In addition, her construction had been subsidised by a loan from the British Government, one of the conditions of which was that she could be converted to an armed auxiliary in times of war. She was listed as such in many recognition guides, but had not actually been so converted. As a result of her cargo of ammunition, she was a legitimate target. The main issue the Allies had with her sinking was not so much that she was an illegitimate target, it was the manner of her sinking. The rules governing commerce warfare at the time had three conditions for sinking a legitimate but unarmed target like Lusitania: a ship could not be fired upon without warning, she could only be sunk after a search for contraband found such material, and if the ship was sunk, civilian passengers and crew had do be placed in a position of safety, which did not include lifeboats. Lusitania was sunk with no warning, no search, and no attempt to place the passengers and crew in a position of safety.

I think I'd quibble that the May 1915 sinking of the Lusitania was certainly a crisis in German - United States relations that almost led to war, and to an extent it was a cloud that hung over both countries' relations, especially when Germany announced its unrestricted U-Boat campaign in February 1917, which saw the severing of diplomatic relations.

But in the chain of events, it really seems like it was the interception and publication of the Zimmerman Telegram, and the public confirmation of its authenticity by Arthur Zimmerman in March 1917, that led to the US declaration of war on April 6, 1917.

I guess let me put out this question: do World War I historians actually think that the sinking of the Lusitania was a major contributing factor to the United States entering the war almost two years later?