The Zimmerman Telegram (or note) had a big effect on WW1 because of how it affected the US. Before the note was sent and intercepted, the U.S. was allready starting to look at Germany in a non-positive light, with such events like the sinking of the Lucitania as one of the main points that added to public anger towards the German government. The Zimmerman Telegram was the tipping point, there was a large public outcry when it was published in the newspaper and a month later the U.S. entered the war. The U.S. sent young, fresh troops that were all but rare in Europe as most of them were killed in the early years of the war. Remember, the U.S. entered at the end of the war, but it's fresh troops helped with breaking the stalemate that was going on for the majority of the war and eventualy Germany was defeated.

If the Zimmerman note was never intercepted, would the U.S. still have gone into the war? Most likely, but the note helped speed up the process so to say. The Zimmerman note had a huge effect on the overall American perception of Germany.

Sources:

"firstworldwar.com." First World War.com. N.p., n.d. Web. 30 Jan. 2014. http://www.firstworldwar.com/features/zimmermann.htm.

"The Zimmermann Telegram." <i>National Archives and Records Administration</i>. National Archives and Records Administration, n.d. Web. 28 Jan. 2014. <http://www.archives.gov/education/lessons/zimmermann/>.