Why did America get into World War 1?

America got into WW1 because the German set up an unrestricted submarine warfare zone, and warned everyone, but America disregarded it. A British ship, the Lusitania, was hit by three German missles, and 128 American citizens were killed. It was a passenger ship, and the German s claimed it was carrying fire arms from America to the UK, wich later proved to be true.