With the end of the war and the signing of the Treaty of Guadalupe Hidalgo on February 2, 1848, the Cortina family estates were divided by the new frontier, leaving a vast portion of their lands inside United States territory. Cortina became an important political boss for the South Texas Democratic Party, and although the new local authorities invalidated many of his land claims, he remained a large rancher. Many landowners of Mexican ancestry suffered from this situation as well, and eventually Cortina came into conflict with an influential group of lawyers and judges of Brownsville, whom he accused of expropriating land from Mexican Texans or ""Tejanos"", who were unfamiliar with the American legal system. "Flocks of vampires, in the guise of men," he wrote, robbed Mexicans "of their property, incarcerated, chased, murdered, and hunted them like wild beasts". Cortina's own skirmishes with the law steadily intensified, and he was indicted twice on charges of cattle theft. However, he was not arrested due to his already considerable popularity among the poorer "Tejanos", who considered this attempt to be nothing but another demonstration of legal harassment by the ""Anglos"" (the Texans of American origin) to their class. With the self-appointed purpose of defending the rights of this social group, Cortina gathered, trained and armed a private army, and on many occasions he used this force to resist the eviction of "Tejanos" from their lands. As a result, he became a popular leader among the poorer local population, many of whom considered him a hero against the abuse of power by the "Anglos".