The Mexicans received the "Zimmermann Note" from Germany. Germany tried to pressure Mexico into the war by attacking US. If the US was distracted by war to the South, then they couldn't be as effective in Europe. However, Mexico declined and didn't enter the war. Check out http://en.wikipedia.org/wiki/United_States_in_World_War_I for more details.
i heard that even though mexico didnt buckle under the pressure put on my germany. Some of the citizens decided to rebel against the U.S at home in mexico, and there was even a few mexican nazis even up to the end of WWII.. just random useless knowledge