From 1836 to 1848 when Cortina was 12–24 years old, parts of the Cortina Grant North of the Rio Grande River was in the disputed territory between the Rio Grande and the Nueces Rivers, claimed by both Mexico and the Republic of Texas. The situation had a big impact on Cortina and his perspective on government and power. When the United States defeated Mexico in the Mexican–American War in 1848, Mexico was forced to cede the disputed territory to Texas. Cortina opposed this concession. However, Cortina's Mexican militia was easily defeated and forced to flee into Mexico when the Texas Rangers, the United States Army and the local militia of Brownsville, Texas and Matamoros, Tamaulipas all organized and fought against his forces. According to Robert Elman, author of "Badmen of the West", Cortina was the first "socially motivated border bandit," similar to Catarino Garza and Pancho Villa of later generations. His followers were known as the "Cortinistas."