How did the German U-boat sinking the US passenger ship the Lusitania cause them to go into world war 1?

The Lusitania was torpedoed by a German U-boat on 7 May 1915. It was a contributing factor to the US entering WW1.

The Germans were conducting unrestricted U-boat warfare, and the Lusitania, a British passenger ship, was deemed a fair target because it was flying the British flag. Lusitania went down in approx 18 minutes killing 1,198 people (128 Americans).

This incident caused a storm of emotions over what was seen as the slaughter of civilians. In both England and the US, the press stoked Anti-German sentiments further causing a large backlash in the US against Germans. This one incident helped sway public opinion, which had been largely neutral, towards the Allies and away from Germany.