German Foreign Secretary Arthur Zimmerman was seeking a military commitment from Mexico, so that Mexico would battle against the United States if the US entered into the Great War.
The United States had almost been ready to join the war (which we now call World War I) after the Lusitania was sunk in 1915 -- but Germany then pledged an end to its unrestricted submarine warfare, and the USA held back. But when the Zimmerman Telegram was revealed in 1917, just as Germany resumed its submarine attacks on American merchant ships in the Atlantic, the US did proceed to enter the war.
