What was revealed about Germany when the British intercepted the Zimmerman telegram?

When the British intercepted the Zimmerman telegram, they realized Germany was offering a diplomatic proposal for Mexico to become their ally during World War I in case the United States were ever to take up arms against the Germans. However, this outraged the American public including the president at the time which was Woodrow Wilson, and this led to speeding up of American armament and the declaration of war on Germany.