Which event was most influential in turning American public opinion against Germany?

Most
sources indicate that, even though the US did not join the war until a few years after the fact,
the event that turned American public opinion against Germany was the sinking of the
Lusitania in 1915. The Lusitania was a luxury British
steam-liner, and among the 1,198 that were killed, over 100 of them were US citizens. This
wanton slaughter of civilians completely outraged many countries in the world, and America was
no exception.

Submarine warfare was a completely new innovation at the time
and brought along with it technological implications that completely circumvented the
"Prize Rules" which governed the guidelines of wartime ship capture. America and the
rest of the world saw this violence not only as cruel but almost uncivilized, and American
public opinion was rapidly swayed to a position that was against the Germans'
plight.