Here's What You Need to Remember: What Mexico could do was tie down American troops and equipment that otherwise would have been sent to Europe. Not that many U.S. troops would have been needed to stop a Mexican invasion, though recent history warns that many, many troops would have been needed to occupy Mexico. But a second Mexican-American war could easily have triggered a disproportionate response, as the American public demanded that the troops stay home and defend the nation.

It was a little over one hundred years ago when Mexico almost invaded the United States.

In January 1917, German foreign secretary Arthur Zimmermann dispatched a coded telegram to Heinrich von Eckardt, the German ambassador to Mexico. With Germany locked in bloody stalemate with the Allies in France, and Britain’s naval blockade strangling the German economy, Kaiser Wilhelm’s government was about to make a fateful decision: declare unrestricted submarine warfare, which would allow U-boats to sink merchant ships on sight.

That also meant sinking the ships of neutral powers, most especially the United States, which would likely respond by declaring war on Germany. But Zimmermann had instructions for his ambassador: “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.”

This was the famous Zimmermann Telegram. Decoded by the British, who passed it on to  the Americans, it became a justification—along with unrestricted submarine warfare—for the U.S. declaration of war on Germany in April 1917.

In the end, Mexico turned down the proposal. But what if Mexico had declared war on the United States?

In fact, Mexican president Venustiano Carranza did order his government to study the German offer, according to Friedrich Katz, in his book The Secret War in Mexico. Carranza’s decision wasn’t surprising. In Mexican eyes, the United States had illegally seized one-third of Mexico’s territory during the 1847 Mexican-American War, including what are now the states of California, Utah, Nevada, Arizona and New Mexico. In 1916, a U.S. Army expeditionary force had entered Mexico in pursuit of the notorious revolutionary Pancho Villa, who had raided American territory.

However, when Mexican officials studied the proposal, they concluded that Germany would never be able to ship sufficient munitions (especially given the inevitable American blockade), and that annexing three U.S. states would lead to permanent conflict with America. Ironically, given the current furor over Mexican illegal immigrants in the United States, the Mexican government worried in 1917 that adding millions of Americans to Mexico’s population would mean that Mexicans couldn’t be sure “whether we had annexed them or they had annexed us.”

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As Katz, an Austrian refugee from Hitler who became one of Mexico’s most important historians, put it: “All these reports show that Carranza did not want to rush into war with the United States, and certainly not on the basis of a German offer of Texas, Arizona and New Mexico. But it can also be surmised from these indications that he wanted to keep Germany in reserve for the eventuality, which Carranza considered a probable one, of an American attack on the Mexican oil fields.”

So what if Carranza had decided to ally with Germany and attack the United States, either to recover lost territory or to preempt a feared American seizure of Mexican oil? In 1917, the Mexican army numbered perhaps sixty-five thousand to one hundred thousand soldiers. In 1914, the U.S. Army had just ninety-eight thousand men. By the end of 1918, it had swelled to four million, of which two million had been sent to France. America also had tanks and aircraft (provided by the British and French while American industry geared up for war), a huge navy and plenty of money.

Short of Kaiser Wilhelm’s spike-helmeted legions storming New York and Baltimore, there was no way Mexico could seized the southwestern United States. Yet this didn’t matter to Germany. What Mexico could do was tie down American troops and equipment that otherwise would have been sent to Europe. Not that many U.S. troops would have been needed to stop a Mexican invasion, though recent history warns that many, many troops would have been needed to occupy Mexico. But a second Mexican-American war could easily have triggered a disproportionate response, as the American public demanded that the troops stay home and defend the nation.

And that’s where history might have changed. The focal point of global events in 1918 was France and Belgium, not Mexico or Texas. Russia, gripped by Bolshevik revolution, had pulled out of the war by 1918, leaving Germany free to transfer fifty divisions from the Eastern to the Western Front. In spring 1918, the Germans launched a massive offensive in France that nearly won the war.

What helped revive the exhausted British and French armies were the divisions of fresh Yank troops streaming off the transport ships and into the trenches. If those troops had stayed in America, it is possible that World War I might have ended later than it did, or perhaps even in a compromise peace instead of a German defeat.

Fortunately, none of this happened. In the end, the Zimmermann Telegram did accomplish something: it hastened Germany’s downfall.

Michael Peck is a contributing writer for the National Interest. He can be found on Twitter and Facebook. (This first appeared several years ago.)

Image: Wikipedia.