I think it is great that we live in a country where we celebrate holidays with great gusto that are not ours. It does disturb me that the majority of Americans probably do not know why Cinco de Mayo is important in Mexican heritage.
Allow a very short Mexican history lesson: after the Mexican-American war, France occupied Mexico. The president of Mexico at the time decided that the country could not afford to repay its debts and asked that the country as a whole be allowed two years not to make payments. The president also promised that the debts would be repaid. This request was unacceptable. So, in 1862, France began to march. Remember France had fought on the side of America and wanted America to become a larger power than it was in 1862.
Cinco de Mayo marks the victory of the Mexican Army over the French at the Battle of Puebla. It was really a symbolic victory bringing unity to Mexico as within three years, the French defeated the Mexican Army and placed a ruler of their choosing in power.
There you have it, a short history lesson.