Key Point: There was never potential for a Mexican invasion of the United States.
It was 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.