The Zimmerman telegram was a huge diplomatic mistake by Germany.
Step-by-step explanation:
In the period of World War one Germany had put an attempt on sending Zimmerman telegram into Mexico. In the telegram there was a clear message to invade over the United states.
It suggested an alliance between Germany and Mexico during World War 1. This was the reason America joined the War. Germans deliberately made the decision because the Germans were aware about United States attacking Mexico and not getting involved in World war 1.
And if the Americans would make any mistakes than Germans were surely going to win the war.