Indeed. It is a complete sentence, since there is no slang or incorrect grammar in this sentence.
Answer : The details in this passage lead to the inference that the British waited until they were sure the telegram would encourage the US to enter the war. 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 US was close to getting involved in the war, but he waited until "February 24, when Hall sensed that the Zimmermann telegram would tip the balance in favor of the U.S. joining the Allied forces." Because Hall waited to share the telegram, a logical inference is that he wanted to be sure that it would be effective when he did .
