France wanted an empire to park its extra pretenders on, and staked out a new empire.
Late in 1861, a well-armed French fleet stormed Veracruz, landing a large French force and driving President Juárez and his government into retreat.<13> Moving on from Veracruz towards Mexico City, the French army encountered heavy resistance from the Mexicans near Puebla, at the Mexican forts of Loreto and Guadalupe.<14> The 8,000-strong French army attacked the much poorer equipped Mexican army of 4,000. Yet the Mexicans managed to decisively crush the French army, the best army at the time, and one that had not been defeated for almost 50 years.<15> This happened on May 5, 1862.<16>
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Some historians have argued that France’s real goal was to help break up the American Union, at the time in the midst of a civil war, by helping the southern Confederacy:<23> “The Mexicans had won a great victory that kept Napoleon III from supplying the confederate rebels for another year, allowing the United States to build the greatest army the world had ever seen. This grand army smashed the Confederates at Gettysburg just 14 months after the battle of Puebla, essentially ending the Civil War.” The consequence of Cinco de Mayo to the United States has been thus recognized:“The defeat of the French army had consequences for America as well…the French defeat denied Napoleon III the opportunity to resupply the Confederate rebels for another year.”<24>
Donald W. Miles adds, “At the time, there were fears in the United States that the French would use Mexico as a base to back the Confederacy, so President Lincoln and his secretary of State went out of their way to appear ‘neutral’ in the Mexican situation. They did not want to take on the French and the Confederates at the same time“.<25>
Its importance to the young nation that was engaged in Civil War is obvious.
The usefulness of Mexican defeat of the stronger European nation made things much easier for the Union. If their wars hadn’t weakened Mexico to the point it defaulted on its debts, though, the whole crisis could have been avoided.
http://my.firedoglake.com/ruthcalvo/2011/05/05/cinqo-de-mayo-the-holiday-for-ending-wars/