As the Lusitania neared the end of her crossing, a German U-boat sank three British ships in the waters south of Ireland through which she was about to sail, and he [Lusitania Captain William Turner] received repeated warnings that U-boats were active on his intended course. Yet on May 7, as the Lusitania entered the most dangerous part of her passage, Captain William Turner actually slowed down, apparently worried by patchy fog.
In fact, Turner was ignoring or at least bending every one of the Admiralty's directives for evading German submarines. He was steaming too close to shore, where U-boats loved to lurk, instead of in the relative safety of the open channel. He was sailing at less than top speed, and he wasn't zigzagging (later he claimed to believe that zigzagging was a tactic to be adopted only after a U-boat was sighted).

The lost of the Lusitania provoked great outrage in the United States and helped create the climate of public opinion that would later allow America to join the war. It also marked the end of any delusions that the "civilized" manners of 19th century warfare could survive into the 20th.
http://www.pbs.org/lostliners/lusitania.html