On September 16, 1810 he proclaimed independence from the Spaniards. Around the 1800's mexicans, especially whites, had grown to a recent rule. Instead of celebrating a mass, Miguel Hidalgo issued a cry for revolution. It sparked a 10 year old war that finally ended Spain's rule. 

When Mexico won it's independence it was a huge country, about twice it's present size. Its northern boundary claimed Texas, New Mexico, Arizona and California, also parts of Nevada, Utah, and Colorado. These northern parts of Mexico were undeveolped or unexplored. Very few Mexicans lived in the north. High authorities in Mexico city didn't have control over this big territory. As the United States expanded Mexico's people coveted the fertile lands of northern Mexico.  Soon Mexico got smaller and the United States claimed the land.