It's a fake.

That's what everybody thought at first. Why would anyone suggest that Mexico attack the U.S.? That's just bonkers. Despite being technically neutral, the U.S. still had a formidable military. Really formidable compared to Mexico, which had recently gone through a revolution and was in the process of writing a new constitution. And Japan attacking the U.S. was double bonkers. They were allied with Britain—you know, the opposite side of the war from Germany.

A lot of stuff about this telegram seemed fishy.

The experts at Room 40 bent over backwards to verify to the U.S. that it was the real deal. And some people still didn't believe it until Zimmermann himself admitted to writing it.

In the century since the Zimmermann Telegram was revealed, the implausibility of Mexico invading and taking back whole states has only increased. This is probably due to the way that the World Wars solidified the U.S. as a mega power and for Mexico…not so much. Sorry Mexico, but you be you.

While historians may list multiple causes of the U.S. entering the war, the Zimmermann note tends to top these lists. It was certainly a major factor that swayed public opinion and even helped change the president's mind about going to war.

Amazing how one little note could move people more than speeches, news articles, or even propaganda. Today the Zimmermann Telegram is recognized as just the right message at just the right time to send a nation to war.