Which of the following doctrines did president James Polk use to justify sending U. S. troops to the rio Grande at the start of the Mexican-American war

In taking this initial action to begin the Mexican-American War, President Polk was affirming the Monroe Doctrine, which was a line of foreign policy in which the U. S. would oppose any foreign colonialist activities in the Americas. It was also an offshoot of the doctrine of Manifest Destiny, which posited that America had the right to unbridled western expansion.