The Republic of Texas was a sovereign state in North America between the United States and Mexico that existed from 1836 to 1845. Formed as a break-away republic from Mexico by the Texas Revolution, the nation claimed borders that encompassed an area that included all of the present U.S. state of Texas, as well as parts of present-day New Mexico, Oklahoma, Kansas, Colorado, and Wyoming based upon the Treaties of Velasco between the newly created Texas republic and Mexico. The eastern boundary with the United States was defined by the Adams-Onís Treaty between the United States and Spain, in 1819. Its southern and western-most boundary with Mexico was under dispute throughout the existence of the Republic, with Texas claiming that the boundary was the Rio Grande, and Mexico claiming the Nueces River as the boundary. This dispute would later become a trigger for the Mexican–American War, after the annexation of Texas.