Germany promised to give Mexico back land they had lost previously, which was New Mexico, Arizona, and Texas.
The Zimmermann Telegram is known to be one of the main things that made the United States join World War I. President Woodrow Wilson did not want the United States to join World War I, as he felt that it would harm the country and the people. The Zimmermann Telegram was one of the last straws for Wilson, along with the continuing unrestricted submarine warfare coming from Germany.
Germany sent an encrypted note to Mexico, now known as the Zimmermann Telegram. This note got intercepted by Great Britain though, so it actually never reached Mexico. This note stated that if Mexico attacked the United States, they would get land back that they had lost previously. This land is known as New Mexico, Arizona, and Texas.