Final answer:
The Zimmerman Telegram proposed a military alliance between Germany and Mexico, offering the return of lost territories to Mexico if the US entered WWI, leading to outrage in the US and the subsequent declaration of war on Germany.
Step-by-step explanation:
The action proposed by the Zimmerman Telegram that led to the US declaring war on Germany in 1917 was a military alliance with Mexico. Germany promised Mexico the territories of Texas, New Mexico, and Arizona—lands it had lost in the Mexican-American War—if Mexico allied with Germany and the US entered the war on the side of the Allies. The British intercepted and decoded this secret communication, and upon its revelation to the American public, it spurred outrage and was a significant factor that motivated President Woodrow Wilson to ask Congress to declare war on Germany, which they did on April 6, 1917.