While the immediate cause of the war was the U.S. annexation of Texas (Dec., 1845), other factors had disturbed peaceful relations between the two republics. In the United States there was agitation for the settlement of long-standing claims arising from injuries and property losses sustained by U.S. citizens in the various Mexican revolutions.
Another major factor was the American ambition, publicly stated by President Polk, of acquiring California, upon which it was believed France and Great Britain were casting covetous eyes. Despite the rupture of diplomatic relations between Mexico and the United States that followed congressional consent to the admission of Texas into the Union, President Polk sent John Slidell to Mexico to negotiate a settlement. Slidell was authorized to purchase California and New Mexico, part of which was claimed by Texas, and to offer the U.S. government's assumption of liability for the claims of U.S. citizens in return for boundary adjustments.
When Mexico declined to negotiate, the United States prepared to take by force what it could not achieve by diplomacy. The war was heartily supported by the outright imperialists and by those who wished slave-holding territory extended. The settlement of the Oregon boundary dispute (June, 1846), which took place shortly after the official outbreak of hostilities, seemed to indicate British acquiescence, for it granted the United States a free hand.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.