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Support for the war

Besides alleging that the actions of Mexican military forces within the disputed boundary lands north of the Rio Grande constituted an attack on American soil, the war's advocates viewed the territories of New Mexico and California as only nominally Mexican possessions with very tenuous ties to Mexico. They saw the territories as unsettled, ungoverned, and unprotected frontier lands, whose non-aboriginal population represented a substantial American component. Moreover, the territories were feared by Americans to be under imminent threat of acquisition by America's rival on the continent, the British.

President Polk reprised these arguments in his Third Annual Message to Congress on December 7, 1847. He scrupulously detailed his administration's position on the origins of the conflict, the measures the U.S. had taken to avoid hostilities, and the justification for declaring war. He also elaborated upon the many outstanding financial claims by American citizens against Mexico and argued that, in view of the country's insolvency, the cession of some large portion of its northern territories was the only indemnity realistically available as compensation. This helped to rally congressional Democrats to his side, ensuring passage of his war measures and bolstering support for the war in the U.S.