Final answer:
The Zimmermann Telegram proposed that Mexico aid Germany if America entered the war by encouraging Mexico to invade America to regain lost territory. This proposal was intercepted by British intelligence officers and contributed to the tensions between the United States and Mexico.
Step-by-step explanation:
The document that proposed that Mexico aid Germany if America entered the war is the Zimmermann Telegram. In January 1917, a German official suggested to the German ambassador in Mexico that Mexico should be encouraged to invade America to regain the territory taken in the Mexican-American War, if the U.S. entered the war. This proposal was intercepted and decoded by British intelligence officers and further contributed to the tensions between the United States and Mexico.