It was immediately clear that the telegram was of inestimable value in finally drawing the United States into the war on the Allied side, a long-time British objective.
Still, there were problems to be solved before the message could be shared with the United States government. First, Room 40 was one of the British government’s darkest secrets. Its existence as the source of the compromise had to be concealed from the Germans. Likewise, the British needed to conceal from the Americans that they had been reading American traffic, a touchy issue in that the United States was a neutral country. Third, the telegram still contained a few gaps that might lead the Americans to question its authenticity or actual meaning.

Hall hit upon an ingenious idea that addressed all three issues. Using a contact in the Mexico City telegraph office, he was able to obtain a copy of the enciphered message that had been forwarded from the German embassy in Washington. This version had been sent using code 13040, since the embassy in Mexico did not hold code 0075. As an older and less sophisticated code, the British had recovered most of it and were able to read virtually the entire text, allowing them to fill in remaining gaps.

In addition, as a forwarded message, the telegram had been given a new date and header information by the Washington embassy. Use of this version would allow the British to convince the Americans that the message was obtained in Mexico and lead the Germans to suspect the same. Room 40’s role would therefore be concealed.

Source: NSA.gov