When World War I broke out the United States declared its neutrality. That was a popular position among the American people who had no interest in being drawn into the war. Great Britain however, was Americas most significant trading partner, and the US had every intention of continuing to trade with it including selling it arms.

World War I also did not stop the travel of Americans and Brits from crossing the Atlantic. The British Cunard line continued to sail its large liners between the Liverpool and New York. The RMS Lusitania was commissioned in 1907 and was the fastest liner in serviceable to maintain a speed of 25knots. While the Germans had begun to use submarines against British shipping, the Lusitania was confident that it was fast enough to outrun any German sub.

In late April the German Embassy published the following ad:
NOTICE!
TRAVELLERS intending to embark on the Atlantic voyage are reminded that a state of war exists between Germany and her allies and Great Britain and her allies; that the zone of war includes the waters adjacent to the British Isles; that, in accordance with formal notice given by the Imperial German Government, vessels flying the flag of Great Britain, or any of her allies, are liable to destruction in those waters and that travelers sailing in the war zone on the ships of Great Britain or her allies do so at their own risk.
IMPERIAL GERMAN EMBASSY
Washington, D.C. 22 April 1915

1,265 passengers and a crew of 694 embarked on the ship when it departed New York's Pier 54 on May 1st. In addition to the passengers, the ship also carried in its freight holds munitions for Great Britain. As the boat was nearing the British Isles, the Royal Navy became concerned about its safety and sent destroyers to escort it. The ship refused to communicate with the Royal Navy, however, and the escorts ships never intersected with the ship. On April 30th the German Submarine U-20 left Borkum for the Irish Sea. She attacked a number of merchant ships sinking three of them. The British Admiralty knew she was operating off the coast of Ireland and warned all the ships. The captain of the Lusitania Captain William Turner took what he thought were prudent steps, closing watertight doors and preparing her lifeboats for launching in case there was a need.

By May 7th the captain of U-20 Walther Schwieger decided that since he was low on torpedoes chose to return home. At 12:45 while the submarine was surfaced lookouts spotted a ship on the horizon. It turned out to be a large merchant vessel- the Lusitania. Schwieger ordered the sub close on the target. When the Lusitania was 700 meters away, U-20 fired one torpedo. The torpedo hit below the bridge causing a massive secondary explosion. The captain ordered the ship to head for the Irish coast, but the engines could not respond. He soon ordered abandon ship- while the wireless operator sent out an SOS. The Lusitania sank 18 minutes after being struck by the torpedo. Of the 1,959 people aboard when the torpedo hit 1,195 were lost.

The sinking of the Lusitania was a very controversial event. First according to International Maritime Law at the time it was incumbent on a naval vessel which stops a civilian vessel to allow the civilians to disembark before sinking the ship. However, that law was written before the age of submarines, ships that could quickly be rammed by a ship if it was given warning. The fact that there were 128 Americans aboard the ship turned the sinking into a crisis between Germany and the United States. Although it would be two years before the United States entered the war, the sinking began to turn American public opinion against Germany.