The Zimmerman Note was a note sent from Germany to Mexico asking Mexico to align itself with German interests. If the US entered the war and if Mexico opposed the US, the reward would be Arizona California and one other state -- perhaps Texas. It was a major contributor to the declaration of war on Germany by America.
The sinking of the Lusitania was another major event. When the Lusitania was sunk, 2000 people were affected.
Submarine warfare really aggravated the US. Many merchant ships were sunk.
