Answer:
The Zimmerman Telegram was a message from Germany to Mexico where Germany tried to make an alliance with Mexico in case the U.S joined the war. Germany said that they would let Mexico have their old territory back, which included Texas. This upset many Texans because they didn't want to go back to be under Mexico's control because they had taken a lot of time to gain Independence, and were happy with their lives in America.
Step-by-step explanation: