Cinco de Mayo started with an unpaid debt
The Mexican-American War of 1846-48, along with a civil war in 1848, had left the Mexican government in deep debt. In 1861, President Benito Juarez suspended the repayment of debt to foreign countries for two years while his government attempted to right a foundering ship of state. I guess he didn't know about deficit spending.
England, France (then led by Napoleon III), and Spain said no, non, no and sent troops to Mexico to "encourage" the Mexican government to reconsider its decision. However, the British and Spanish soon realized that France intended to not just reclaim its debt, but actually conquer the country, so they withdrew. The deciding battle, the Battala de Puebla, took place 100 miles east of Mexico City on May 5, 1862, where 4,000 Mexican fighters famously defeated a French force twice its size.
Unfortunately, the French were to return with stronger forces and finally subdue the country, which would be ruled by the French stooge Emperor Maximilian for only a few years before he and his troops were overthrown and the emperor executed.
One reason those of us in the United States might honor those who won the Battle of Puebla is that this victory impeded France's ability to supply arms to the Confederate Army.
Many think today celebrates Mexico's independence, but that actually occurred on September 15th, 1810. Save a few Coronas for that day, too.