the zimmerman telegraph was an important event that upset the US and one of the reasons it went to go fight in WW1. what it was is a /secret/ telegram that the germany communicated to mexico wanting to team up against the US if they ever went to war. if germany won the war then mexico would claim some territory of the US and germany would offer money to it as well. so basically it made USA seem weak because not only was germany teaming up against the US with the help of mexico but it was also resuming sinking american ships.