The British waited until they were sure the telegram provoked the US to enter the war.
Admiral William Hall knew that outrage was growing in America over Germany's announcement late in the day of January 31 that the German navy would resume unrestricted submarine warfare. Hall knew that the United States was close enter in the war, so Hall waited until February 24, thinking that Arthur Zimmermann telegram would make the U.S. join the Allied forces. He showed the telegram without revealing that the British deciphered the German codes.
Zimmermann –German Foreign Secretary- had sent a telegram to Count Von Bernstoff, the German ambassador in Washington, with the intention to convince Mexico to support Germany in the war.
