Even as opponents of the war grew more vocal in 1847, public support for seizing even more territory from Mexico than Polk and his advisers had initially envisioned was rapidly gaining ground. In the fall of 1847, the expansionist press began to call not merely for the acquisition of Mexico's northernmost provinces, but for additional cessions below the Rio Grande. Some demanded nothing less than that country's complete absorption by the United States. The call for the annexation of Mexico was not new. A year before the war began, the Democratic Review had predicted that the United States would one day acquire all of Mexico. Few U.S. leaders gave the idea much consideration, however, focusing instead on territorial objectives that seemed readily obtainable and relevant to national interests.
However, many Anglo-Americans balked at the prospect of assimilating a population of eight million non-whites. Ironically, the racism which had helped to fuel Manifest Destiny also defined its limits. Southern expansionists like John C. Calhoun, who had earlier demanded the annexation of Texas, now scoffed at the idea of granting U.S. citizenship to conquered Mexicans.
In the end, the All Mexico movement faded even more quickly than it had emerged. In February 1848, U.S. and Mexican diplomats reached a negotiated settlement based on the United States' initial territorial demands and signed the Treaty of Guadalupe Hidalgo. Though he now believed that the United States was entitled to a larger portion of Mexico's national domain, Polk was also aware of the risks involved should he reject the product of Trist's negotiations. Having recently gained control of the House of Representatives, the Whigs were threatening to block further military appropriations. Believing that the terms of the treaty were preferable to the unknown consequences of rejecting it, Polk sent the treaty on to the Senate, which ratified the agreement in March, 1848.