Final answer:
Juan Cortina drafted a proclamation defending the Treaty of Guadalupe Hidalgo and resisting oppression after seeing a Mexican beaten by law enforcement in Texas.
Step-by-step explanation:
After witnessing a law enforcement officer beat a Mexican in their Texas town, the bandit who drafted a proclamation that said he/she would defend the Treaty of Guadalupe Hidalgo and resist the oppression of his/her Mexican American people was Juan Cortina.
Juan Cortina, also known as Juan Nepomuceno Cortina Goseacochea, was a rancher, military leader, and folk hero. In 1859, outraged by the abuse of power by some Texas officials against Mexicans, he issued the "Carta de Juan Cortina" ("The Cortina Letter") where he denounced the injustices against Mexican Texans (Tejanos) and took up arms, leading to what became known as the Cortina Wars. These actions aimed to protect the rights of Mexican-Americans and uphold the promises made in the Treaty of Guadalupe Hidalgo after the Mexican-American War.