The significance of the Zimmerman note:

Before the Zimmerman note we had a lot of ties with the British. We wanted them to win the war but we weren't willing to join in the war. The Zimmerman note was the straw that broke the camels back for the u.s. When the Germans sent the note to the Mexico talking about the united states we couldn't take anymore. So we joined the war trying to put a stop to all the madness.