Zimmerman Telegram is one important reason why the U.S went to World War I. Foner talks about how the Zimmerman Telegram was made public by the British spies in March 1917. He says how the German foreign secretary  Arthur Zimmerman call on Mexico to join the war against the U.S and promised to help recover their territory lost in the Mexican War of 1846-1848. I think that Foner basically covered most of what the Zimmerman Telegram was.
A revolution in Russia overthrew the czar and established a constitutional government making it more plausible to believe that the U.S would be fighting on the side of democracy. The war resolution was passed the Senate 82-6 and House of Representative 373-50.