The military conflict that served as a precursor to the Mexican-American War was the Texas Revolution, which took place from 1835 to 1836. This conflict began when American settlers in Texas revolted against Mexican rule, leading to the establishment of the Republic of Texas. The annexation of Texas by the United States in 1845, however, created tensions between the two nations. The U.S. claimed that the southern border of Texas extended to the Rio Grande, while Mexico disputed this by asserting that the Nueces River was the true border. These disputes ultimately culminated in the Mexican-American War in 1846.