What position did President Polk take regarding the border of Texas?

President Polk claimed the Rio Grande as the border between Texas and Mexico, which significantly heightened tensions between the two nations and contributed to the outbreak of the Mexican-American War. Polk's assertion was based on the belief that the annexation of Texas justified this boundary. This position was rooted in the idea that the Rio Grande was not only a more suitable geographic boundary but also the river that had been acknowledged by Texans as the southern border of the Republic of Texas prior to its annexation by the United States. Polk's insistence on the Rio Grande as the border intensified conflicts and disagreement with Mexico, which recognized the Nueces River as the boundary instead.