Final answer:
Mexicans were upset with the United States during the Mexican Revolution because of land dispossession, racial discrimination, and exploitative laws like the Greaser Act. The annexation of Mexican territories led to a lasting pattern of antagonism and injustice.
Step-by-step explanation:
Many Mexicans were upset with the United States during the Mexican Revolution for several reasons, including racial tensions and land disputes that arose from the annexation of Mexican territory following the Mexican-American War. The U.S. government often declared Mexican and Spanish land deeds 'imperfect', questioning their authenticity and causing Mexicans and Mexican-Americans to lose their lands in states like Texas, California, and New Mexico. This culminated in a legacy of violence and racial antagonism that persists to this day.
Revolutionary philosopher Ricardo Flores Magòn was a significant figure, mobilizing Mexican Americans against the Diaz regime and the discrimination they faced in the U.S. His newspaper, Regeneración, and the Plan de San Diego inspired a revolutionary plot to form an independent republic, causing concern among whites in the Southwest.
Racial policies and violence subjected Mexican Americans to discrimination and economic hardship. For instance, the Greaser Act in California disproportionately targeted people of Mexican descent, contributing to their marginalization and exploitation.