The main reason that we saw Woodrow Wilson change his mind and enter the US into WW1 was the continued use of submarine warfare in the Atlantic ocean by the German military. We saw this lead to the sinking of the Lusitania and we also saw an increase in the willingness of the German military to attack civilians. In addition the German government sent what is called the Zimmerman telegram in which they asked for Mexico to attack the US from the south.
