If not for Cinco de Mayo, I might still be living in the CSA*.

[trof]

*Confederate States of America.

Wiki:

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> Dr. Miles goes on to explain that "Napoleon III had hesitated to take on the United States directly, but now the news of the Civil War changed everything". It meant that the Americans would be occupied with their conflict between North and South for some time. Upon hearing the Spaniards and the British had sailed off to grab the customs house in Veracruz to start collecting their duties, Napoleon decided he would not only send the French navy, but would also start looking for someone to place as emperor in Mexico. He would then use Mexico as a base to help the Confederates win their war against the United States. Napoleon saw this as an opportunity not to be missed.<26> Dr. Miles then concludes, "The Emperor of France ordered his generals to spend a few months taking on Mexico and then - using Mexico as a 'base' - help the Confederates win their war against the United States. What if they had succeeded? The United States would never become the significant world power it is today...the Mexicans not only took their nation back, but influenced the outcome of the U.S. Civil War."<27>
http://en.wikipedia.org/wiki/Cinco_de_Mayo#Consequences_to_the_United_States