What doctrine did President James Polk used to justify sending US troops to the Rio Grande at the start of the Mexican-American War?

President James Polk justified sending U.S. troops to the Rio Grande by invoking the doctrine of Manifest Destiny, which held that the United States was destined to expand its territory across North America. He claimed that the U.S. had a right to the land between the Nueces River and the Rio Grande, asserting that American blood had been spilled on American soil after a skirmish with Mexican forces. This rationale was used to galvanize public support for the war against Mexico, framing it as a defense of American territory and sovereignty.