The Zimmerman Telegram - Big Mistake!
In my opinion, if the Germans never sent the Zimmerman Telegram, the Central Powers might have won. Out of all the mistakes in world history, it would be up there as one of the worst. Let's see why.
The Zimmerman Telegram was sent by Germany to Mexico in 1917 during World War I. They wanted to form a coalition with Mexico against the United States. In return, Mexico would regain Texas, New Mexico, and Arizona. Mexico rejected this, but something even more important happened. You see, British intelligence detected the telegram, and they sent it to Washington. (source) The American government was furious. So they declared war on Germany.
Now, why was this a big mistake? Well, you see, the Central Powers were winning until the Americans joined. Romania and Serbia had capitulated by 1917, and Russia was close to losing. (They did leave later on.) Also, Germany was getting close to Paris. They should have just kept the Americans out of the war. This was a very bad move that I would not do if I were Germany.
Here is a translation of the Zimmerman Telegram from Wikipedia (words in parenthesis are mine):
We intend to begin on the first of February unrestricted submarine (Lusitania) warfare. We shall endeavor in spite of this to keep the United States of America neutral. In the event of this not succeeding, we make Mexico a proposal of alliance on the following basis: make war together, make peace together, generous financial support and an understanding on our part that Mexico is to reconquer the lost territory in Texas, New Mexico, and Arizona. The settlement in detail is left to you. You will inform the President of the above most secretly as soon as the outbreak of war with the United States of America is certain, and add the suggestion that he should, on his own initiative, invite Japan to immediate adherence and at the same time mediate between Japan and ourselves. Please call the President's attention to the fact that the ruthless employment of our submarines now offers the prospect of compelling England in a few months to make peace.
Signed, Zimmermann
So now you know about the Zimmerman Telegram. I might make another episode of Big Mistake about World War II. (Yes, the Germans made another mistake there.) So that's it, guys. Bye!