At the beginning of World War I, Great Britain actually had very little to offer France because it was mostly a naval power. Their army in 1914 was probably the best trained and experienced out of all the major participants because of their colonial wars, but it was tiny in comparison to the French and German juggernauts. However it could impose a blockade on Germany and almost immediately wiped the seas clean of German shipping. Initially many thought this to be useless since most believed the war would be done in a few months, but by 1916 whatever food the Central Powers had went straight to the troops and the civilians began to starve.

Germany had one method to end the siege, and that was the U-Boats or basically an early submarine. Sonar hadn't really been invented yet, so the U-Boats could easily just destroy the most expensive of the British battlecruisers and dreadnoughts. There were only two things stopping them: First, they didn't have enough of them to launch an effective attack, so the Germans waited a year until they produced at least 200 of them. Second, the United States. American citizens and merchants were dying in U-Boat attacks, and the Central Powers feared if they pressed too hard, the United States would enter the war. The US hadn't by this point because it had a long, long history of nonintervention (which is incredible when you think about nowadays) and there was a large Irish and German immigrant population, who were anti-Allied.

The Zimmermann Telegram is what tipped the scales. Arthur Zimmermann, the German Foreign Secretary, sent a telegram to his ambassador in Mexico, basically saying they were going to launch the U-Boats pretty soon. If that pisses off the US enough, this ambassador was supposed to approach the Mexican government and offer an alliance, as well as asking Mexico to help Germany convince Japan to switch sides (at that time Japan was with the Allies). Most explosively, Germany said it would help Mexico reclaim Texas, Arizona, and New Mexico. The idea was the United States would be too busy fighting Japan and Mexico to help France and Great Britain.

At the beginning of the war Great Britain cut all of Germany's telegraph wires, so they had to employ American ones to send out foreign correspondence. The British figured out a way to tap into that and consequently were pretty much reading all of Germany's top secret messages. Once they revealed what Germany had planned, American citizens were outraged and President Woodrow Wilson, who hemmed and hawed about entering the war, finally did.

What makes the Zimmermann Telegram interesting is its timing. Without that impetus, Wilson may have dithered longer in deciding to mobilize or he may not have at all. If he waited just a few more months, Russia would've fallen to the October Revolution and Germany could turn its undivided attention to the Western Front. By then, even if the United States wanted to help the Allies, by the time it trained and got its troops over there, France and Great Britain may have lost. And then you have to ask all these hypothetical questions, like with a German victory would Hitler have risen? Would World War II have happened? Or paradoxically, would a dictator have appeared in France instead thanks to the reparations demanded by Germany? It's funny how one document changed the course of history.