Final answer:
Zimmermann sent the message through the U.S. State Department as an expedient alternative after the departure of the German U-boat was delayed, and he used it as a channel considering the U.S. was still neutral. However, this action contributed to the U.S. joining WWI after the telegram was intercepted and made public.
Step-by-step explanation:
Arthur Zimmermann, the German foreign secretary, resorted to using the U.S. State Department to send the Zimmermann Telegram because the designated German U-boat was delayed and he became impatient for the message to be delivered. The telegram was an integral communication proposing an alliance between Germany and Mexico should the United States join World War I against Germany. The allure of reconquering lost territories such as Arizona, New Mexico, and Texas was offered to Mexico in return for its cooperation. Transmission via the U.S. as a neutral country was seen as safe, and President Wilson had allowed messages to such officials as a courtesy, despite the U.S. claiming to be neutral in the conflict. However, the scheme backfired disastrously when British intelligence intercepted and disclosed the secret telegram to the Americans, fuelling public sentiment against Germany and contributing to the U.S. decision to join the war.